That's what 'bitcoin-brokers.org' does.  I used their service for a while with very good results, but now I just use Coinbase (in part because I've given up all hope of financial privacy using Bitcoin.)  I've followed them since, and so far I've not seen anyone with bad results.

The escrow agent could always decide to pack it in with all current money or get hit by a bus or whatever of course so, as always, it's best to never play with more than a guy can afford to lose.

A while ago the operator stopped taking on sellers when he had to many (and started again as conditions change.)  That was a good sign in terms of gauging his motives, but of course it could also have been a confidence trick.  One way or another if I were running a legitimate escrow, I would not want to be holding more value than necessary.  To actually do it right in terms of security and fail-safe systems and such could take a lot of focus and be a fair amount of work.

I'm guessing that the fun will come to an end if/when it becomes unlawful to deposit cash into a random person's account without some modicum of 'know-your-whatever-ness'.  I'm surprised it's even allowed now to be honest.  Probably it would be implemented by some combination of claw-backs and/or banks just shutting out customers who do this.  My guess.