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!
That's basically a non-starter for me as I need to give all potential users a good UX.
Although, it'd make more sense if we tell them that and save you the time to reply to a few customer support tickets. Will add it to our roadmap :)
That's also a non-starter because that defeats the purpose of a VPN.