If the purchase and swap would stretch the budget, but you feel it's still a good deal, get it and hold it. You could always sell the transmission if you only want the engine, recoup some of the cost and apply those funds towards what you'll need to mate the Magnum to the LS3.
It's possible the seller needed the ECU for whatever he's replacing the LS3 with?
Lots of questions for sure, but it sounds like you may have found a good deal if it's legit. Just remember you don't HAVE to swap it immediately. But if you decide to, I have an engine lift if you need to borrow it.
