A case of trust, communicate with them and see how long they have been around here for.. if they are known by many people and is a regular member, I wouldn't worry as much as a newbie who just racked up 50 posts and starts selling anything and everything..
Its really a case of judgement and doing the right homework. I've bought from user forums tons and tons of times and never had any problems, other than sometimes a wait for things to be psoted, as I appreciate it is difficult to get to a post office if you are stuck at work from 9 til 5 when the post office opens at 9 and closes at 4 or something like that..
Also what I personally prefer is to get bits from local people so I can actually drive and pick it up and pay cash there and then, not only does it mean you see if there and then to make sure everything is in order, but also they get the cash and no paypal fees etc, and also it allows you to mingle with fellow owners who share the same love as you do and can pass on advice or ideas etc etc..