The way I see it is looking at your advert, you mention one wheel needs a refurb (£100), you mention one dent (£70 dent removal), scuffed bumper (£100 smart repair), new mot shortly (£40), £200 for some road tax in next couple of months. That's just over the £500 you were willing to come down, so stick to your guns at £12500.
Good luck I hope you get what you want, selling privately can be a headache don't get hung up trying to please the first person who comes along wait for the third/fourth/sixth whatever and sell to them.