If memory serves from when I upgraded a few years ago, you can upgrade to Windows 8 from Vista. Then you could upgrade from that to Windows 10 provided there free upgrade still works. They officially ended the free upgrade a couple of years ago, but the last times I've tried it, the free upgrade still seems to work.
I don’t think selling an OEM Vista Disk is a good idea.
In my opinion it’s best installing Win 10 from an usb drive and activating it afterwards. You can find Win 10 OEM keys for under 20 Bucks but it might happen that one does not work. So it’s best to just look up how your reseller handles this.