If it is someone that I am not familiar with, I click on their profile and look at their most recent posts. I like to buy from people who don't only make an appearance in the classifed threads. If they have been on the site for years and are frequent posters in the pow wow, trad history and collecting etc forums, that carries weight with me. Most people here, like me, find this forum far too valuable to risk losing it over taking somebody on a bow.
Another thing you can do is to ask the seller for references (other people on the site he has dealt with). Many times if it is a new person and I ask for references the seller will mention someone from the site that I do know; then I just ask that person.
Good luck.