If you're suggesting that an illegal activity is happening, you need to report it to the appropriate LE. If they get caught and you did anything to help them in even the smallest way, you're are an accomplice.
We have zero tolerance for suggesting or condoning illegal activity on this forum.
Any post suggesting that the illegal activity is acceptable or encouraged will be deleted and the member reprimanded.
Otherwise, except for the serial numbered part, the receiver or trigger groups, there is no part of a gun that is traceable or would ever legally require it to be destroyed.
That requirement for being destroyed would only apply to fully automatic receivers and a few of their parts.
It is perfectly legal (under federal law, state laws may vary) for a person to manufacture their own firearms, from scratch, and no serial number is required. It can even be sold. It can NOT, however, be manufactured for the PURPOSE of being sold unless the person building it has the appropriate manufacturers license from the ATF.
If your friends are taking serial numbered receivers and deserializing them and reselling them as function guns without serial numbers, they are committing several federal felonies. They are not the type of person that I would call a friend and, frankly, I would judge you by the company that you keep.