The single most important thing anyone interested in getting into the art market can do is research. Assuming that you have a rough idea of where your center of interest lies, it's vital to learn all you can about the period, styles, manufacturers, etc. This can be done online, through books and of course visiting museums. It pays to see first hand and if possible handle as many examples of genuine piece as possible so as to to get a feel for them. Such experience can prove invaluable when accessing the authenticity of a work.
It is always preferable to trust one's own knowledge than to be forced to rely solely upon the descriptions provided by the sellers or auction houses.
As a buyer, it is important to always keep your wits about you.
Whilst there are plenty of nightmare stories out there, it's important to remember that not everyone is out to swindle you. There are two different factors to take into account: intentional rip-offs and errors in description. Whilst the former is clearly done with the malicious intent, the latter is generally more widely spread.
Simply put, not everyone is an expert. Even those who make their living off of antiques and who have been in the business for decades don't know every single aspect about every type of antique from all over the globe. They are, as a result, going to make mistakes. It's important to note that in many cases, auction houses will not accept returns, especially if there has been an exhibition prior. The same is true for yard sales, flea markets or for those visiting France "Brocantes" (not the be confused with Antiquaires). The buyer is solely responsible for what they have bought.
Watch out for "bargains"
It's probably incorrect to say that there are no bargains anymore but there are certainly fewer around.
Much of this can be attributed to the development of the internet over the past 10-20 years. This has enabled more and more private sellers to research and potentially get their items appraised meaning that adverts and individuals claiming not to know what the item is should be viewed with a certain amount of suspicion and objective reasoning. A typical red flag would be phrasology such as "I found this in a box my grandfather's attic".
Equally it now means that potential buyers from all over the world can view these items be it from online auctions and retailers or galleries and antiques shops who had their own websites. As a result, it's fair to assume that in most cases, the apparent bargain will also have been noticed. If nobody else is bidding or appears to be interested, then it might beg the question why. In such cases, it is best to ask one's self: "What have all these other people noticed, that I haven't?". This is of course, where genuine expertise comes into play.
Generally, it can be assumed that most types of antique have a standard price (for example an older well carved Netsuke will cost upwards of 100£, you're unlikely to find a decent wakizashi for less than 300£) anything which is priced massively below this should raise some alarm bells.
Forgeries, reproductions and copies
First of all, it's important to take note of the difference between forgeries and copies.
A forgery is created with the purpose of fooling the buyer into believing that they are purchasing a genuine piece.
The sad truth of the matter is that many forgeries are actually sold as such in their countries of origin, if only for legal reasons (antiquities, for instance, by law, cannot be removed from their countries of origin). Issues arise when either tourists or unethical dealers purchase them and bring them back to their home countries. In the case of private individuals, generally its their descendants who end up believing they have a genuine piece simply though lack of knowledge. There are certain outlets however who are notorious for selling these fakes and describing them as authentic.
It is therefore vital to research companies and dealers thoroughly before a purchase through forums and reviews. People are generally very keen to highlight trouble sellers. For sites like EBay, don't rely solely upon feedback as collectors who have recognized fakes won't tend to buy from them and therefore can't leave feedback. Instead, do a search for the individual's pseudonym. One fake can't quite often hide many, especially if the dealer is supposedly specialized in the domain.
Be extremely vigilent as far as laconic or vague descriptions no matter who they are from. Generally, you can envision returning an item only if it doesn't fit its description, otherwise the purchase is made at the buyer's risk. As such, some descriptions are extremely non-commital.
Two phrases to look out for are "In the style of" or "after". These mean that no exact attribution has been given and that the sellers are reluctant to put their necks out. This isn't necessarily a sign of anything crooked going on but it should be noted that any seller reluctant to give a firm and clear description can tell you two things: either they lack knowledge about the given field or have some doubts concerning authenticity, etc. In either case, the buyer needs to be aware of the risks involved.
Concerning damage, this depends a lot on the context. Some sales, especially yard sales, many dealers and even some auctions rely upon those taking part in having attended an exhibition or viewing of the items. As such, it's up for prospective buyers to take note of the condition. Generally however sellers online or in auctions are required to explicitly mention any faults, failure to do so should entitle the buyer to ask for a refund. To get round this, some sellers will list items using the following expressions: "please form your own opinion from the photos". Usually, this can be taken to mean that there is something, somewhere which has caused the seller to be unwilling to give a guarantee.
The same is true for poor photos. At best, an image which is unfocused, poorly lit and/or blurry indicates a seller who has little to no interest in what they're doing. Such individuals can potentially be extremely problematic to deal with as they're likely to put about as much time and effort into everything else as they did their photography.
Estate sales don't guarantee authenticity
One of the favorite expressions of dealers, especially on EBay, when describing an item's provenance is "from the estate of a deceased collector" or sometimes "an old soldier/diplomat/adventurer". Not everything that is old is necessarily valuable or rare. To put it bluntly, even older people bought cheap copies or forgeries.
The seller should be able to provide some information regarding the estate and its previous owner. Auction houses are usually only too keen to champion the provenance of certain items, especially when they come from a recognized and celebrated collector.
Keep in mind that the original owner could have died five months ago and have still been buying items off of EBay last year.
Certificate of authenticity red flags
In all likelihood, if you've done a decent amount of searching for antiques, the chances are that you've come across items being sold with "Certificates of authenticity". Sadly, a huge amount of these aren't worth the paper that they're printed on. They range from the laughingly bad (full of grammatical errors and obviously mass produced) to as convincing as the fakes themselves. The simple fact of life is that anyone can type up one of these certificates, claim to be an expert or a professor and sell them off accordingly. The quality of these fakes range from laughably poor to indistinguishable with the "best" sometimes even stealing genuine signatures from authentic organisations and individuals.
Certain categories of antiques are far more prone to this than others. Generally, the more frequently faked the type of antique, the greater the temptation to fake the papers. Typically items such as antiquities from the ancient world and some types of Chinese art.
In some countries, such as France, some antiques dealers known as "Antiquaires" are legally obliged to provide the buyer with a certificate clarifying that the item was bought from them, for how much, providing a description and guaranteeing their authencity. If the item were found to be a fake in the next 30 years you would be able to ask for a refund.
When purchasing an item with papers, always do your research. Ensure that the person or organisation that has provided the papers actually exist, are certified experts and recognized in their field. It might also pay to contact them, to ensure that they did indeed provide the cerificate.
Condition is key with many items especially ceramics, glass and paintings. In some cases, even the slightest scratch or smallest chip can literally halve the value, if not make it worthless. Ideally, a prospective buyer will have had the opportunity to see and handle the object themselves. Failing that, the description and images provided are everything. If either are lacking, ask for more. If the seller is reticent, then be extremely weary.