You don't. You just tell the truth. A true story. I had a one of those "vintage reissue" Strats, this one came from Fender with a twisted neck...discovered of course after the warranty had expired (there's a whole 'nother story here, but that's for another time). Anyway, when it came time to part with, I put it on consignment with a local shop with the provision that every prospective buyer be informed up front. Lots of lookers, it was a very nice looking guitar, and they ran when told. Eventually I traded it to the shop for stuff and called it a day. About a year later I get a call from a fellow who's a student of a player I regularly accompanied. He says that he's having issues with his guitar and that the teacher recognized the guitar as my old one. He had one question: Was the shop aware of the warp/twist when they acquired it from me? Apparently the shop failed to mention it when he paid full value for it. But wait...! There's more!!!! Turns out this fellow is an attorney. He asked if there was anything that might corroborate the idea that the shop knew... I faxed the consignment agreement to him. The shop did not have a good day, week, month, year because of their dishonesty. Apparently this attorney made it a mission to extract justice...slowly, in the icy cold of space....aka civil litigation. Moral of the story. Be honest, take the hit, live happy knowing you did the right thing, and avoided unpleasantness.