Is a faint smile upon hearing a story from priests about selling lands in Paradise considered a form of mockery of religion, and what should I do about it?

This action is not considered mockery of religion; rather, it is mockery of priests and the ignorant people who believe them. Mockery is a lightheartedness and sarcasm that involves belittlement, which did not occur here. The questioner should ignore these obsessive thoughts, for the cure for obsession is to completely turn away from it, because responding to it increases it and leads to madness. Obsessive thoughts are from Satan, who seeks to lead the believer astray, and fighting them is considered a jihad against the enemy of Allah.