From the listing:
"The car has the typical Healey rust to the outrigggers, floors, part of trunk floor, rockers, etc. Cross member looks solid. Some rust through on frame where floor meets top of frame."
If they mean that the frame rails have rusted through this could be a serious problem. The frame would be weakened by such damage and may sag, causing the fit of the doors to shift.
You need to have pictures of the rusted areas, and it'd be best to have it looked at, if possible....