Scamadvisor is pretty good, but I'm not sure if it lists black market sites.

However, the easiest way to avoid scams is to use your brain. A black market seller of no doubt counterfeit items (if he even actually delivers) asks for no escrow and you give him $300 in an untraceable currency? How long did you think about this? Unless you're buying drugs or child porn there's really no reason to be buying stuff off the darknet. It's much safer to use a legit site.