you can always use a multi signature bitcoin address for this purpose. from what i understood a 2 of 3 signature is what you are looking for.
buyer and seller and you each have 1 key. the transaction will require 2 signatures to be valid. when the trade completes one other party signs the transaction with his key and then you sign it with your key and the trade will be complete.
if you want to be paid in this process you can choose a 3 of 3 one then all parties have to sign the transaction which pays the seller and your fee in one place.
check it out:
https://en.bitcoin.it/wiki/Multisignature
