MGB281

GC
If you have no success with the supplying dealer then IMHO then you have to bite the bullet and carry out the repairs, my reasoning is this, you have paid top of the market price to a dealer but as a private seller in the winter you will never recoup much more than half that. Ellisojo has shown how easily it is to do an excellent DIY repair (metallic silvers are notoriously difficult to match) and if your rust is similar or not too much worse then now is a good time to learn "how too". If the rust is more extensive on the sills then these panels are your saviour; https://www.mikesatur.co.uk/MGTF/Body-Repair-Panels/
I have repaired the rear sills on my TF with them, I cut the rusted metal out and had a someone weld the new patches in. With Nightfire red you can barely see any miss matched colour even with a Halfords aerosol. I can't think that the problem is that great otherwise you would have noticed it pre purchase although a dealer can oftem make it look better than it is. If you post photos then you will receive better advice.