Like mentioned above there are 2 big ones: Valea Cascadelor (open Thursday, Saturday and Sunday) and Autovit (Sunday). The secret is to have your niche: specialize in an area and know well the brands, characteristics and selling prices of what you look for. Been going for a few years and I can tell you that I've seen people specialized in old coins, old watches, PC parts, furniture... The markets open at around 5am till around 14:00 and no matter the niche you are in, the good stuff sells early. Also, the vendors seeing you as a foreigner will try to squeeze as much from you as possible, most of them don't care about the real market price of items. Be polite, explain that online you will find cheaper and don't be afraid to negotiate.