Fraud is a big problem in the theme world.
I haven't had a chargeback for years by doing a few things...
Reject sales from certain countries without exception.
Must be a live real site, if that is not provided as asked on our Order form, the order is rejected.
Paypal account must be genuine verified (this is better now than it used to be years ago).
Name must match with Paypal account - IP must reflect the Paypal address.. ie: IP puts the buyer in country X but Paypal has them as a resident of country Y (good chance it is a cracked Paypal account and will chargeback when the owner realises)
Ask Questions - send the purchaser a PM asking a few questions -
Look for obvious clues - anyone in a hurry is automatically flagged as being a little suspect, pushy agressive "where's my skin" type posts two seconds after purchase, highly likely to be a fraudster looking to get the skin and skedaddle before you realise, leaving you with a chargeback a few weeks later, and your skins warezed all over the net.
Be smart, if it feels odd, it probably is, if it is too good to be true, it probably is.
What a lot of resource providers are doing now is asking customers to verify they are a customer here, by using their xenforo.com account to post in a customer forum.