I would charge thousands for a purchase escrow agreement that held up in all 50 states...and everytime there was a problem claimed with a product or deal count on at least a few more K before the holder would know what to do (I would go mandatory binding arbitration as opposed to investigation or court action)
If you found a lawyer cheaper he wouldn't be worth hiring for it.
Escrow gets expensive, even when the holder gets to keep the interest.
You could just pay the start up for the agreement and esign it on each transaction (figure 10k or more for start up, and many hours per week depositing funds, notifying sellers to send, getting okays from buyers, and sending funds)
You could then have the mandatory arbitration be AAA small claims, costs and fees shared equally by the parties, but the costs would be so high that most parties would have to settle.