Check with the seller that the bike comes with the same warranty that would be applied to a new bike. For some brands/models, that's a "lifetime of original owner" on the frame against faults. If the bike shop is nominally the First Owner then you might get less, or even no warranty.
Your example bike has a RRP of 900 UKP. Based on that you're saving 245 pounds, or 27% which sounds reasonable.
However RRP is rarely the sale price. A quick google returns Sigma Sports selling the same bike for 649 UKP which is very slightly under your cost. You're paying more than retail for a used bike.
However if the bike is not otherwise available, then paying "over the odds" may be required. Only you can decide if this is reasonable.
Check the brake pads and tyres for wear. They should be almost brand new. Tyres should have sprues and even traces of the mould flashing visible, indicating under 10 km of travel. Brakes should function perfectly.
A minor scratch or ding may be acceptable in the paint, but anything that shows carbon fibres or metal may be too much.
I can't really comment here but if the bike is designed for off-road, how much force/impact has it already taken?
Some bike shops hire out their bikes, vs your description of being a showroom bike only. Personally I'd always try and take a bike for a test ride outside, which may explain the traces of road-dirt.
On the positive side, you'll find all the cables should be well bedded-in already.