The kind of escrow you're talking about might be a good idea for very high value books, but realistically nobody is going to buy one of those from a completely unknown member anyway.
For lower values, some combination of boardies vouching for each other, Paypal protection and use of common sense usually works OK. There have been some instances of boardies being actually scammed and losing money without any resolution, but given the number of sales that take place on the boards, both publically and via PM, those are few and far between.