If as you say the car has been well maintained with paper work, & you have given details as truthful as you can, you have done your bit. was the car serviced at your local garage? if so the persons that bought it may have seen your car in for any faults & may have enquired, you sound a decent seller so I would not offer any refund as I think you WILL HAVE TOLD them of any faults the car has / had if any. there aren't many sellers like you Jos I can assure you.