RE: Sounds like a BAD deal to me......
If it were me I would pass. I'd rather have an unused engine in my shop for sale to a known buyer than the real risk of the alternative, as seen here plenty of times in the past unfortunately. No offense intended to any innocent potential buyers out there of course, just a reaction to past precedent. It's a risk vs. reward thing. If it were truly worth it to you, would you have asked?
Good luck with whatever you decide, no right or wrongs, just opinions until the deal is done I suppose.