I’ve been buying and selling on eBay on and off for many years but I’ve never had an issue with a buyer until now so I don’t know much about the process.
I sold a dress last week that was brand new with tags and in perfect condition (I’m extremely fussy so know it was in perfect condition!) but as soon as I marked it as dispatched the buyer decided they needed it by today. They sent quite a few messages about it which made me suspicious they were hoping to say it didn’t arrive in time. But it was tracked so as soon as it’s arrived today they’ve filed a dispute asking for £22 back out of the £40 price because there’s ‘make up on the dress’ 
The message I received from eBay said ‘xxx has filed a payment dispute outside of eBay about an order that was placed on 08 Jul, 2021. This means the buyer has disputed a charge directly with their payment provider.’ What does that mean, charged it back through their bank? And why would someone do that rather than use eBay’s resolution process?
I sent one of my photos of the dress as asked but haven’t been shown any photos of the imaginary make up stain. I suspect they’ll just wear it then add their own for the photo but where does that leave me? Can they just keep it for less than half the value or do they have to return it to get a full refund instead?
Sorry for the long post, I just don’t know how I can prove anything and it’s really frustrating if the buyer will get away with it.