If you're not haggling at a market, you're overpaying, according to the Consumerist. Shopping at the markets can net you great bargains, but there's an etiquette to it. The first thing you should keep in mind is that vendors add in a "haggle factor" to the price, so you should definitely try to ask for a lower price (vendors interviewed at the Brooklyn Flea Market say 20-25 per cent off is reasonable, though some shoppers say they start at half off).
Another tip for haggling — politely — at the markets: don't offend the vendor by asking what his or her best price is and then proceeding to keep haggling after that.
For more tips, check out the article at the Consumerist. Do you haggle at the flea market (or garage sales, etc)? Share your own strategies in the comments.
Tips on Haggling from the Particular People at the Brooklyn Flea Market [The Consumerist]