About 20 years ago I bought a railway-style petrol nut runner from a well-known internet auction site. It came from a seller in Ashford, Kent, which I later learned was then (whilst HS1 was being built) the national hotspot for railway plant theft. There were many convincing reasons for believing it was a genuine sale from an honest seller, and the serial number was clearly visible on the tool (and shown in the auction listing).
Not long afterwards I was contacted by BTP who were investigating a spate of rail plant thefts and, it turns out, the person from whom I bought the tool, and I arranged to take the tool to show them. They had no evidence to prove that the machine was stolen and there were no reports of that serial number having been nicked, but they were interested in the seller, who was known to them. The advice from BTP was that buying it in good faith, and having it in my possession and using it was not a problem, but if I were to sell it and it turned out that it had been stolen then potentially I could be charged with handling stolen goods, even if I didn't know that it had been stolen. After a cursory check whether the serial number was on their hotlist they showed absolutely no further interest in investigating that particular tool. They did say that they would get in touch if there were further developments, but I never heard any more from them.
As for the tool, ironically that was stolen about a year later during a break-in at the heritage railway where I was working. The theft was reported to local police and BTP who issued a crime number and as far as we can tell then did absolutely nothing else.
My feelings based on this experience and what BTP said at the time is that even if you are suspicious about the provenance of the machine but can show you have taken the reasonable steps to check if it is stolen, then you are unlikely to be charged. However if it does turn out to be stolen you run the risk that it will be returned to its owner and you will be left out of pocket.