Some of these threads are scaring me a bit.. But then someone has to be trusted at one point. Are group buys safer?
I'd simply suggest to use escrow from a very high-trust person like John K, who I think will even do m-of-n escrow transactions to almost eliminate third-party-risk.
Another great security is to ensure you do business with people relatively local in case you need to collect through the legal system or appeal to relatives of the person -- at the very least, people within the same jurisdiction as you (and confirming ID is great -- a rather ghetto way of doing this, without relying on scanned gov't IDs, as example, would be to send a postcard with a code on it to the person's alleged address and have them relay the code to you when they receive it).
There are plenty of places to gather an idea of someone's trustworthiness, from
WoT to the forum's rep system. Bitcoin (as an "ecosystem") has tons of functionality for determining trustworthiness and doing low-trust or no-trust transactions, but you need to be diligent. Even the largest-by-volume exchange, MtGox, has a terrible habit of fiat withdrawal blackouts of weeks, or in the current case, over a month - so certainly any company or individual can fail to meet advertisements, and that's why it's a great idea to get paranoid.
I've used John K but I've not understood this forums huge drive towards escrowing everything with him. It seems like the easy way out of doing any sort of due diligence. Seems to be the exact opposite of what bitcoin is all about. It's like John K became PayPal of this bitcoin forum.