It is quite possible the seller was ignorant of what he had and wasn't intentionally lying. I've encountered this scenario many times, so don't write the person off as a liar.
Society Finches are a completely domestic finch not found in nature. They have no breeding season, breeding whenever. You are right, they do not molt the tail all at once; more likely his tail went to build someone else's nest. The good news is that a healthy finch will regrow the tail in 3 to 4...