I think you should settle for a reasonable discount against open market value, something like 5%.
You won't have the hassle of marketing, cleaning up for sale, an empty period before new owner, less risk of sale falling through, etc. They may want to take furniture too which a third party almost certainly won't.
Start with a few valuations and take it from there. It might be easiest for both parties to negotiate via solicitor (but this will cost more).