If the boiler is nicked and the serial number is one the wanted list, it does not matter where you bought it from, the police if notified are entitled to call on you to investigate and return the boiler to it's rightful owner.
If after investigation they decide you have knowingly purchased stolen goods you matey are in bother.
If the boiler is returned to its rightful owner you will be out of pocket.
Stolen is stolen doesn't matter if its a car, 1/2lb chopped mince or a vailant boiler.
Well that's just an assumption. We have no idea how the boiler was acquired. All I know is I paid £820 cash and I own it
Let's be realistic - how many of the items on eBay or gumtree do you think have been stolen? Do you honestly think that the police will go out and check and investigate each stolen item. I think the police have got more important things to worry about than a (what we assume) is a stolen boiler?