Hi,
Thier web site doesn't give much away about the company and they do not appear to be listed as a Limited Company at Companies House. This would help to gauge how deep their pockets might be.
Best bet would be to call them and sound them out on pricing. There are quite a few "supplement" domains on Sedo but none with pricing.
If you want to make a quick cash in on your registration then ask for £500 and see what they offer.
If you are prepared to scare them off and keep the domain then ask for £1,000 and see what the response is. We see proteinpowder.com is for sale for $2,000 so you are unlikely to get more than £1k.
How did they find the domain for sale?