I've never heard of gold parties, but apparently they're a huge scam. If you want to sell jewelry, find a jeweler--preferably more than one--and get a price for it intact. The workmanship has to be pretty shoddy for the gold to be worth less as jewelry than melted down.
Which is pretty much common sense, as is the notion that you should check the price online before you sell. So why does this work? Because people trust friends and acquaintances more than some stranger in a shop; if you're in a home, you feel more trusting. The parties often involve alcohol, which is not a good negotiating tool. As the poor labor market drags on, I expect that this sort of thing will only get more common.