Depends on what you're buying at auction.
British car auctions now own WebuyanyCar. Anything you sell to WBAC is turned out at a BCA auction.
If you get a BCA assured car, these are generally ex-lease vehiles and come with warranted mileage,an AA mechanical and condition report and you can claim your money back if it's misrepresented or breaks down within 48hrs. They generally have a full service history and are in decent nick. Youre not getting any more than that out of a dealer if buying a used car.
Last year I'd a 59 plate 143 S which had a winter pack, and I'd remapped to 181bhp/288lb/ft and 93,000 miles full service history and new springs/shocks/tyres all round. I'd had this since Feb 2011, I sold this in May 16 to replace with the other Exeo. There was no interest whatsoever at £4,000, or when reduced to £3,500 so sold it my brother in law for 3 grand.