It's a good price for the spec on a carbon frame if it all stacks up in terms of condition, he's got the receipt with an address matching his, check it over thoroughly for damage to frame, bottom bracket
Won't get much else in terms of carbon and 105 for that. I'd maybe offer £550 or £575 if it's the right size, colour etc for you
Ask him for a test ride
There's a Dolan in the classifieds with SRAM rival. Which also looks a steal
Advantage of both is you won't see many others around on the roads if any, a bonus in my book