Why don't you use a trustless escrow service like one of these:
- bitrated.com
- bitescrow.org
- coinb.in/multisig/
If you like I can act as an arbitrator (it's technically impossible that I run away with your money)
I can only +1 this suggestion. This is bitcoin, why fiddle with this cumbersome trust stuff.
For a gentlemans bet, you can just use 2 of 2 multisig and not even use arbitration.