why not stick it on pistonheads or ebay as needing headgasket, may fetch ok money if your lucky, cheaper than your planned repair and then selling??

A small car dealer will offer sod all anyhow and most likely be looking to sell it on the lot, so may well check it carefully - they will spot the signs.

A larger dealer may be less fussy as they will be giving bottom book price and chopping out to auction. But then you would be buying a car off them so you'd be paying out on forcourt prices.