If I was selling an engine, no way in hell i'd let just anybody with a toolbox come 'inspect' it's innards. No way in hell.
The gods honest truth is, when you buy a used engine, you just don't know what you're getting. Even if the engine comes with a 'guarantee', that is no guarantee it isn't going to be a piece of crap once you get it installed.. and such guarantees rarely cover or compensate you for the time spent putting the engine in, and taking it back out.
Like the local junkyard wanting to sell me a tranny for $800, with a 30 day 'guarantee'. That just meant if the tranny puked 30 days after I bought it, the guy would give me my money back.. but not the time, money, or effort it takes to pull the old tranny out, put theirs in, then take theirs back out.
My advice is if you're looking for a good engine, then buy one cheap and either pay someone reputable to rebuild it, or rebuild it yourself.
As far as buying a used engine, never buy an engine you can't hear run first.. unless it's REAL cheap.. and then, only buy it if it turns free with the plugs out.. turns with some resistance with the plugs in, and you do all the normal check things, like checking the quality & condition of the oil.. coolant.. look under the valve cover, that sort of thing.
