If you both trust each other then you can just send the bats at the same time. If not, you could pay via Paypal G&S for the bat - when the bat is received then they pay you via Paypal G&S.There is also a middleman system, you could either send both bats (or similar monetary value) to somebody who then verifies both are as described and in good condition etc.
Has anyone been involved in a bat swap? What's the best way to initiate one and where? And what's the best way of ensuring fairness, if there is anything other than simple trust?