Edit: It's actually legal for the seller in the EU, their EULA doesn't supersede the law. I don't know if this applies to the UK since we left .
https://www.computerworld.com/artic...oftware-licenses-is-legal----even-online.html
https://www.techdirt.com/articles/2...-even-if-software-company-says-you-cant.shtml
Those links refer to retail licences, the ones you're seeing on eBay for cheap aren't retail. They're either MSDN or volume licence, so neither of those links apply.