The fastest way I can think of to become trustworthy is to have personal info on file somewhere.
Personal info doesn't mean much when it comes to a global community with magic internet money. Even if I know who and where you are, it will be very hard to use legal means to get back money if you scam. And after all that hassle, the collected evidence may not even be enough to convince a court.
Apart from the whole trust-part, there's another problem: how will you decide who's right and who's wrong if the buyer claims to have received a brick instead of the ordered items? A proper escrow would personally inspect the items before forwarding the package to the buyer.
I've read a general rule about acting as escrow, and I like it: don't offer it until many people have asked you to escrow for them.