Despite what many will say, it's completely legal, EU highest court ruled that companies can't forbid the transfer/re-sale of legitimately owned software licenses.
All the legitimate ones guarantee that the license will work and they will replace it for free if you have any problems with it.
There are big online computer shops which also resell licenses. They have names, addresses, thousands of customers. If you have a problem with a second-hand license bought there you can make a consumer complaint to the authorities.
Really? I’ve seen OEM Windows licenses for about €15 on a German website (IIRC the practice of selling them was specifically declared legal in Germany), but Office went for €50 there.