Could just be he's realistic or perhaps not bothered about squeezing a few more quid out of the deal, as with any car purchase you want to check all the paperwork matches the car, so service history has to be present, mileages lining up with MOTs, receipts for work done, replacement parts etc help show if the owner looked after the car. Also watch for Category D insurance as there are a few cars out there with low prices which then turn out to be damaged/repaired.
Ideally do a history/finance search and if that looks good get a mechanic to check the car out, if the chap has owned the car for a few years then his demeanor could be a good insight into the car being well looked after.
Good luck and don't rush into it, there's always another one out there despite what the seller will tell you
Tom