If JohnWittle is the only bidder in the auction, then in a second-bid auction he would receive gwern's soul for free,
Well, yes, technically that's true... but what prevents/discourages gwern (or his accomplice) from submitting an $N-1 bid (where N is the current sole bid amount)?
Typically, second-bid auctions are sealed, and all opened at once at the end of the auction, so it won't be known that JohnWittle has bid, or how much he has bid, until the auction is over.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.