There could be many many reasons for ths and only diagnostics will tell you. The list is endless but I'll give you a few that coud be causing it.
Drivers airbag, knee airbag, passenger airbag, seatbelt, pre-tensioner, Airbag ECU, brake swith, speedo, coil spring . . . you sort of get the picture that there are many things it could be. The fact that the seller has said it was a damaged repaired one but nobody has been able to get rid of the light makes me think that you should probably walk away.
MultiECUScan will most likely give you the answer. For a cheap price, I would take a chance as I've done these numerous times before but I think you should think twice as it's an MoT failure at the very least.