This is a murky area unfortunately, there is no clear cut answer to this question. Though it is incredibly hard to tell what the true laws and legislations are concerning this topic, it is safe to say, that you can find out if you look up your specific state laws as well as federal regulations.
There is no specific federal gun laws that say you can or cannot build an AR-15 and then sell it yourself, no matter what method you choose to build your weapon.
The wording of the legislation that IS in effect currently is confusing and convoluted but, it is safe to say that double checking with your state is always the safest and smartest avenue to take when anything is in question concerning firearms.
Ignorance is not an excuse, and there are heavy fine and jail time in these circumstances, and these are both things that I’m quite sure no one wants to endure or deal with.You can typically bring your lower into your local firearms dealer and they can instruct you on how to complete the necessary paperwork and process for registering your weapon for a serial number.
When it comes to the ATF, they have stated on their website: “If an individual is engaged in the business as a manufacturer or seller of firearms then that person must obtain a federal firearms license.”
But how does that concern the average, everyday person? It comes down to intention. If you built your AR-15 with the specific purpose of selling it, then you must obtain a permit for selling firearms, aka a federal firearms license.
Though, on the other side of that, if you built your AR-15 and intended to keep it after building it, and a year or so down the line you decide you don’t like it and want to build another, you can sell the original AR after you have registered the AR. AGAIN check your state laws on the resale of your AR some states make it rather easy but some can be quite difficult.
I’m sure you can see how that can become a murky topic, because when it comes down to intention, how is anyone to actually know? As the average weapon owner, no one wants to go through the hassle of obtaining a federal firearms license. There are a heck of a lot of hoops to jump through that are simply not worth the time.
So it can almost go without saying, that yes it is legal to sell your AR-15, but make sure that you have all of your ducks in a row, and that they all coincide with the federal regulations along with your state specific laws.
There are many websites that can tell you the specific legislations enforced in your particular state such as: state gun sale laws. This is a great site that has a lot of information to offer than will answer quite a few questions when it comes to certain state laws.
It is also a good idea to consider talking to your local firearms dealer. They will know the ins and outs of these types of deals, and most owners such as my local shop Relic Hunter, he is incredibly helpful, insightful, and knows the laws for my specific state like the back of his hand!