The education discount is less than the discounts available for purchase on the highstreet anyway. Currys and Costco were doing them at the same price / cheaper than edu store, before any trade-in / cashback offers etc.
It’s not an educational license you just got a discount, I don’t see an issue at all.
CEX, just make sure you take your iCloud off of it.
If you’ve seen something about Apple and reselling of MacBooks bought with an education discount it’s probably something to deter people from buying up devices at a discount and then instantly flipping unopened items for profit. It’s not anything to do with someone buying a Mac for their own purposes, using it throughout their course, and then selling it on some years later.