I'm not convinced there's a real problem that you're trying to solve here. What's an example of something you were going to buy on Craigslist but didn't because of trust issues ?

A substantial part of the second-hand car market is done via anonymous classified, and they're probably the highest ticket item regularly sold on classifieds.

The next tier down is probably electronics, bikes, etc. where a substantial part of the market is stolen goods, where for obvious reasons the sellers aren't going to want identification.