[nostatic]

Flip the script and put yourself in a couple of your customer's shoes:

If I bought a used car from a private party, I'd expect to get the extra key. If I didn't, then used the site and had that same seller now offer to sell me the keys I'd be pissed.

If I bought a used car from a dealer, I'd expect to get the extra key. If I didn't, then I'd still feel like I was getting screwed having to pay after the fact for the original key no matter what the "story" was.

If I traded the car in, I don't want contact with the new owner. If I'm a jerk and withheld the keys thinking I could turn a profit, I'm probably both a jerk and an idiot as the odds of getting a "hit" from the new owner are minuscule

Bottom line is if an extra key isn't involved in the original purchase, then a buy should factor that into the purchase price. You're trying to monetize something that shouldn't even be an issue, and users will be annoyed.