What I can find online says a Ribble Sportive 2018 was £1500 new with Ultegra groupo and Mavic wheels, £999 with 105 groupo. The listed bike is £500 for Tiagra on a 2 y/o carbon frame, with at least scratches to the frame and a ripped hood. I’d say it’s too high a price for that bike, with only that info.
Adding in that it’s on eBay, from a new seller, with no description of exactly how it got damaged, no pic of the non-drive side or close ups of the braking surface... I’d be wary af about it.
That said, I pounced on an Italian road bike that listed not too dis-similar from this ad, and was rewarded with a lovely bike, as described, from someone who seemed legit enough and just didn’t ride it. It was pretty much a gamble and an oddity on eBay.
If you really dig the frame/bike, I’d find a price I was comfortable with risking and negotiate them down to that price. Offers to pay and collect quick work well, as many sellers just want it done. Of course, I’d make sure to check it thoroughly in person, ideally with a witness/voice recording. Remember that possessing stolen goods is illegal, and they can be taken from you by the police/legal owner without compensation.
(Edited for clarity)