Well, I spoke to my sources and it looks like what you described as being missing from the hole on the rear of the block was not a freeze plug (although there are 12 of them as it was explained to me: 2 rear, 2 front, 4 each side). There is one in the position you mentioned, but it is enclosed & covered under a metal shield. The only conclusion we could determine was perhaps it was an actual engine block bolt that failed, which is in that approximate position and is exposed to the elements.
As far as purchasing an engine on E-Bay, I would not. If you cannot see the condition, if you cannot confirm its mileage, that is bad enough. But remember, the vehicle may have been totaled and the engine might be all that was left. With no way to inspect it, I just would avoid it.
I agree with Spacemutt. Rebuilding it is the best option, especially since you know where the parts are coming from. That way, you have an upgraded engine with the history known.
Think of it this way, what I just stated is all you would have to do. If you bought a new engine on E-Bay, first you have to tear it down and then verify the condition of the parts. Then you have to rebuild it anyway. Skip the extra step and save the money, and go with peace of mind.