Someone serious and committed to selling (not just thinking about it) their plate will have the plate taken off their vehicle and put on retention ready to sell easily and quickly.
I'd always steer clear if they haven't bothered to remove the mark from their car, yet advertise it. It shows they aren't organised. And it means you'll definetley have a long wait. Even if it is a genuine buy, the time it takes DVLA to post him the documents after he goes to the DVLA and pays to take it off his car is several weeks sometimes. Then he has to post it to you...
So that's what could have happened in your case, it's possible he has been to the DVLA and even paid them, but sometimes they will take 3+ weeks to send him the new reg for his car and the retention cert.
So as a guideline always buy a plate that's on retention, not one that's still on a car, or be prepared to be patient and very careful.