In a private party sale the seller IS NOT REQUIRED TO CHECK CRIMINAL HISTORY or buyer age etc. etc. It is the BUYERS RESPONSABUILITY TO LEAGALY PURCHAS A FIREARM.
If a convicted felon or underaged person buys from a private party then it is the buyer that is violating the law not the seller.
You are partially correct. If a minor buys and the seller made no attempt (e.g. Are you legally able to have this firearm?) the the seller can be prosecuted.