Model and age might help.
2012 scott spark rc carbon
4 grand bike 13 years old it would be how much it has been used and services. Given it’s a Mountain bike it might be on the high price. Have you looked for similar on eBay.
Pity your not selling it!🤔🤣My reaction would be £800 for a 13 year old bike is a bit of a joke unless it's pristine or perhaps a very rare model.
I'm replacing a nine year old Cervelo C3 in the next few weeks. It was £3200 new and has been well maintained but the signs of nine years use are there. I wouldn't sell her but just yesterday I thought about how much I might get. £250 - £300 if I'm lucky. My bike is "rare" in the sense the colour scheme was only in production for 12 months.
A secondhand bike has far more value to the original owner than the buyer. I wouldn't sell my C3 and will use her for dry weather winter rides. To buy a winter bike of this quality would cost £££££.
The problem for sellers is the item is only worth what someone will pay, not what the seller thinks it's worth.
Model and age might help.