I have one rule when buying used engines: Buy it as a core and automatically assume it will require a full rebuild, and pay accordingly.
It doesn't matter if it is stuck, bars over, or running. I have bought "running" engines that had as much or more internal damage and required more machine work than a stuck engine.
That said, I too have a spare 7.3 IDI stored in the corner of the shop. Supposedly it spun a bearing, but it doesn't really matter. When the time comes, I will build it and fix what I need to, but it makes me feel good having a spare around. It keeps the flathead, 351W, 430 MEL, IH 266 v8, and a DT-466 company.