The point is not whether the combustion chamber has deteriorated in 3 months but did the seller know about its condition before the sale?
If you buy my car off me today I will sell it in perfect working order. If you take it to a garage tomorrow they will probably find faults with it that I don't know about. These faults may have been there for months but I wouldn't be offering any refunds for anything discovered after I sold it.
If the engine was sold with known faults that is a different matter.
I'm also sent the running test video to a guy that was interseted in the engine. He was very happy with me for sending the prove in video the engine is in great condition.
This is a good example. How does a video of it running prove anything (apart from the fact that it did run when the video was taken). It certainly doesn't prove that the internal components aren't worn. If the bearings fail on the first run after the sale and the engine is damaged, would you pay for the repair?