That's a great question!

What we currently do on SellApp, is block purchases coming from VPN's/proxies (via MaxMind) which'll help reduce the rate of fraudulent purchases.

However, that by itself is probably not going to be enough to keep a low chargeback rate for all our sellers, so we are also planning on creating a fraud detection system that factors in a number of aspects in order to keep malicious customers at bay.

For refunds, customers can open a ticket with a seller to request a refund. We also give sellers the ability to set a 'refund timer' and terms of a refund during the product creation process, so that in case something does go wrong, they have that to refer back to.

Lots of room for improvement on both aspects though!