Though I only shop for myself, and am not a dealer when it comes to vintage clothes (the stuff in my etsy shop is just what I once bought for myself but for one reason or another don't want to keep any longer), I too prefer if things are marked with a price - for exactly the reasons that Nicole states. If it's a fair or a flea market, one can always ask if they can do something about the price, especially when buying more than one piece. It's funny, but no marked prices usually also gives me the feeling that prices will be high and possibly above my budget.
My parents and I have a stall at a flea market once a year - just your general, local flea market with a good mix of professional dealers and private sellers like us, where we sell stuff we don't need anymore - household appliances, tableware, non-vintage clothes, books, DVDs, whatever there is. We meticulously price everything the day before. It just makes life so much easier I find. Even though we're usually three or four people at the stand, it sometimes gets so bizzy, with lots of people at the stand (that also means watching out for potential thieves - sadly, every time we hear of something somewhere being stolen, though it has never happened to us) that one wouldn't even have time to think about a price... People ask anyway if something can be done about the price.
Karin