In a flea market, half your work is done if you are buying from a reputable dealer. Identifying one, however, is a task by itself, but it will yield its worth not just in money but also in terms of reduced stress. A dealer's reputation can be checked out in different ways as discussed in this article.
From other Buyers. Be guided by professionals. At markets, dealers who attract crowds are definitely better than the ones which buyers are staying away from. Get opinions from peers through reviews and ratings posted on the sites online.
From Professional Associations. Where professional associations exist, online as well, verify the genuineness of dealers with them. The Chamber of Commerce is an excellent one for such research. Verify from local authorities if there are any complaints against the dealer. Once you have shortlisted a few dealers, check their record with the BBB (Better Business Bureau) on standards and complaints. Manufacturers will have the details on their authorized dealers for the area (get the manufacturer details from the product packaging).
From the Dealer himself. The number of years the dealer has been in business has a direct relation to his reputation. As with years in business, the formal education also implies his better awareness of his obligations and duties as well as the product. Check around the shop and the quality of the goods he displays. Research on these items elsewhere and see how they compare. Ask for references from clients and to speak to some extant or past ones. A shifty one would either not give the details or hesitate at doing so. Ask whether he is a member of any trade association. Ask for valid license or permits as required by law and ensure adherence to all relevant regulatory requirements. Get the business address and contact details and be cautious where a Post Office Box address is given.
Remember, it's always better to err on the side of caution and these due diligence steps will make your whole flea market experience the better for it.