This site is probably the best for trading. I am a member of another online trading group and it is a great group of traders but it is so much easier here utilizing the ORG. Like the OP stated, it does all the pricing and math for you and it's easy to browse and search other potential traders inventory. The only beef I have with this site is when it runs slow. That and I think the subscription rates are a bit high.
My Huge Wantlist: http://www.zeprock.com/WantList.html