This. He has legal rights to what he's already bought with his money. Unless you're a major corporation that can legally prove the seller is squatting with intention to get many dollars from said corporation, you're SOL.
Case in point: I owned a domain I used for my business about four years ago. The registrar bungled up my email when they established my account, so I never got a renewal notice email. The domain expired, and when I talked to them about it they said I had to pay $200 to re-instate the account just to update the email they got wrong in the first place, and then in the course of trying to deal with them they sold it to a third party who wanted $800 from me to buy it back. Needless to say, I simply changed the domain. In a completely, entirely unrelated note, don't touch iPowerWeb with a ten foot pole an always confirm your contact information.