I'm all for buying cracked carbon though usually I go in for cracked tubular wheels. But given that seller seems to know what they're doing, I'd seriously question why they'd sell that frame for £500 instead of having it fixed. I strongly suspect it's as they know it's fucked.
potential bargain tarmac SL5 frameset here - difficult to tell from the pics whether the damage is just cosmetic or something more serious. really just looks like (admittedly bad) paint damage to me
https://www.ebay.co.uk/itm/193697862558?ul_noapp=true&autorefresh=true