If itm is simply not there, and not destroyed and not there...ie, the sump would be full of swarf etc. Then it is possible the engine had a problem, someone took the sump off to inspect, found the damage and decided "fuck that", thew it back together to sell to someone else to deal with.

In this case that seems to be you. I'd be cautious though, if they have done that....there could be other things wrong too so I wouldnt go mad spending money on it. How much are known good second hand engines ?