It sounds like he's hiding something, something to do with a leak. The price is right, and the thank could be ok even if you need to make some minor repairs yourself. The real question is this, WHAT DO YOU WANT TO DO?
I would buy the tank and repair anything needing fixed...thats me. I can fix stuff. Can you? Do you know how a sump works well enough to re-plumb once home? What if you need to customize the plumbing, you know how or want the hassle?
Dont throw good money after bad, if it sounds fishy it probably is and this guy is getting rid of the tank for a reason.
And another thing...the tank needs to be full, everything shut off and the sump alowed to fill...then everything turned back on and let run back to normal...simulating a power outage, water change, etc.
If he wont do this, get your deposit back.