Which begs the question - how did other escrow "agents" get started in the biz. Sounds like a complete catch 22. Did they just do some private deals and get a positive trust rating first? Then move into the escrow biz?
How does anyone gain trust? You do it other ways by doing other deals. If people see you have sold items on here and/or are a long-standing active member of the community, you'll be more 'trustworthy' than a guy who's been around for a week and done nothing other than ask to hold onto peoples' money for them.
If you wanted to gain trust another way and prove you're legit you could offer to scan in personal documents, ID, proof of address etc to give people a little bit more trust in you. I think this would give people a bit of piece of mind, but I'd just go for the building up trust over time route. It's not easy to do so, but that's how it should be.