Hmmm. Depends on who I'm selling it to. I mean, if it's going to some omni-present, all-powerful entity that, for reasons I can't fathom, really wants my soul, I'll have to ask it what it wants it for first and what might happen if I were to go through on the deal. Barring all that, I'd have to ask for a quiet house far away from any of the current insanity that is civilization with a full garden, several hundred trees, solar powered independence, and the means to defend it all. Preferably with lightning.
If it's just an average guy with a gun who wants my soul, I'll just trade it for a wallet full of fifties and a bottle of top shelf rum. I don't know what he'd do with it, but it can't be much more than I'm doing with it now.