Yeah, never had any fear participating in group buys.Even if the person organizing a group buy has few rep, usually their post count or activeness in the community can reassure any doubts. Plus they have a lot to lose if they're collecting payment from so many of us

. I don't think you should be worried about these at all - and if you are, just check out the users and see what kind of reputation they have on the forums. I would say most of them have already done a group buy in the past or are trusted members.
As for classified... don't send payment as gift if you don't trust them. Pay the 4% on your end and it will save you from getting burned. If you sell something, you should almost always be getting tracking as it is, but definitely so if you feel like the person on the other end might cheat you.
But so far I've had nothing but pleasant experiences here on Geekhack. Some really great people here who are very considerate when using classifieds. We all share the same addiction (err, love) for keyboards.