Why is it not plausible for him to actually be doing a favour for his friends? I mean if his friend asks a favour of him, why not? You saying he will push illegals thing, is a different matter entirely. Obviously, no one is naive enough to do so. In this case, with the object being a fake gibson guitar, i see no harm in his actions. Anyone stupid enough to buy believing it is a real guitar without verifying its authenticity, deserves to have a lesson learnt. As far as I can see, there are only 3 groups he can implicate.
1) People who do not know a single thing about gibsons
2) People who are doubtful and bother to authenticate and verify.
3) People who immediately are able to determine it being a counterfeit.
And i do see your point, of him having the responsibility to know what he is selling. But in my opinion, he's just doing a favour for his friend. Nothing wrong, and he doesnt deserve being called a moron. Judging from your reputation in SOFT, i guess you are at least a few years older than the seller, and a few years ( if not more ) wiser.
For the skeptical who believe he is making up stories. I dont see a motive or a reason to do so. For those who see a motive or reason, please do enlighten.