Welcome to the pin-sanity!!!
I have been a member of Pinpics since 2000, and actively trading on the site since 2003. I would wholeheartedly endorse Pinpics to use for trading. In my 11 years, I have only had about four trades go bad. As MAL said, you should always use caution in trading -- if it's too good to be true, it probably is. But for the most part, those who use Pinpics for trading are a wonderful lot of people (met my best friend through a random pin trade over 8 years ago).
Trade Assist enables you to imitate trade offers to others. You identify a pin you would trade away and those you would accept in return. The Trade Assistant then sends emails to those that match (want your pin and have your wanted pin(s) for trade). Using the "What could I get for this pin?" feature is likely not the best way to judge what to trade or accept for a pin. Use your own judgement. I am not sure what criterion Pinpics uses to "match" pins up (I think it may be just LE), but I prefer my own method anyway -- cost, LE, trade ratios. You will develop your own criterion.
As for references, there is a place to check them and post them --
http://www.rnrcollectables.com/referencemain.htm. You can get the link there by going to the "Forum" on the main page, then there is a link at the top for the "Reference Center". You will need the other person's email to use the system. I rely pretty heavily on this site when considering trading with someone I previously have not traded.
If you have any more questions, just ask away. We're all here to help.
Bobbie