Having worked as cabin crew and now check in staff, it is good to see that the check in staff are told only to allow people who do fit the ABP description to purchase such seats. Anyone who asks for an exit seat as they have a broken leg or bad back are told the reason why they can't have that seat.
As crew, the efficiency of the check in staff in this matter is much appreciated as it saves a lot of problems once the pax have boarded.
As for selling them, I don't see a problem with this, so long as the person buying the seat is told of the responsibilities that go with the seats and does indeed fit the description of and ABP.
Plus, it will encourage people to check in early to try and get these seats, minimising the risk of last minute pax delaying the A/C!