How can I unlock a second-hand iPad mini?

You seem to know your position with this unfortunate situation. If you are going to buy a used device you have to do it in person and make sure the person is doing all the things in this support article --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

I think I have seen people say something about applying to Ebay about this kind of thing, but I haven't bought anything on Ebay in 6 years, and certainly not anything worth stealing. I could see it would be hard for them to determine if a device was stolen. They cannot see it. So in this scenario it really is buyer beware, unfortunately. I would automatically presume anything being sold is stolen unless you can meet the in person and make sure they have disabled Find My at least.

Having had your own device stolen, you may well have bought somebody else’s stolen device.

Either avoid buying from sellers that can easily disappear, or require a massive discount on the price to make up for the purchasing risks.

Buying from sellers that can’t disappear, or that have an escrow, or from Apple authorized resellers or Apple of course, is more reliable.

The All Too Common SAD Reality of Buying a used iPad/iPhone - Apple Community

Lots of purportedly-Apple pieces and parts being sold are counterfeit, too.

MrHoffman wrote:

Lots of purportedly-Apple pieces and parts being sold are counterfeit, too.

I believe Apple is now starting to deal with that and are featuring activation lock on individual components too.

Apple to expand repair options with support for used genuine parts - Apple

Limnos wrote:

MrHoffman wrote:

Lots of purportedly-Apple pieces and parts being sold are counterfeit, too.

I believe Apple is now starting to deal with that and are featuring activation lock on individual components too.

I was referring to counterfeit “AirPods” and “Apple” cables and such, but yes, the latest updates mean Activation Lock now requires the original Apple Account to release the locks placed on specific iPhone and iPad internal parts as part of repairs.

Non-genuine parts have been widely available of course, which then causes some discussions around here about warning messages about displays and batteries.