It is happening more then most think. A few seem to do a better job then others. They part out mix masters and then use those parts to assemble what appears to be complete carbines.
Getting a book or two can help. But it would not be the save all. Newer people don't always have the expertise to search out the replaced items. As always it's buyer beware. Pictures don't give you the same opportunity to look it over like having it in hand. One reason to ask about their return policy.
Depending on how the weapon is priced, you might be able to take a chance. But if it gets up into original carbine price range, that would be time to do more homework. There are still gems out there. Hidden in closets that relatives have no clue what they are worth and a great deal might be available. But online sales people know what they have and/or what they have done to it. The old saying it if looks like too good of a deal, it usually is.