I can only think of three things that would cause what you have described, without there being anything wrong with the mower. The first, as you suggested yourself, would be turning it upside down, perhaps to wash it underneath. The second would be over-filling with oil. The third would be tipping the mower to an excessive angle while it was running, so that oil flooded the crankcase breather valve.
If the oil level was correct when you received the mower, my guess would be that the problem had been corrected (oil level returned to correct amount, or mower put back to a normal angle, no longer tilted excessively), and the buyer either did not know what had been done to it, or simply chose to deny everything to avoid embarrassment. However, let's see what other people can suggest.
