AR-15 is semi auto.  There is federal standing on intra-state sales involving non FFL.  In VA for example I could (but likely wouldn't) sell any firearms to anyone* without doing any background check.  It is called a person to person sale.  Only if the sale crosses state lines would it fall under federal law.

* You are prohibited from selling to a person KNOWN to be prohibited buyer.  If the person is anonymous well there is no way for someone to know if they are prohibited or not and the law doesn't require any "due diligence" beyond that.

For the record I don't think it is smart to sell firearms to other people anonymously just because unless you purchased the weapon anonymously if it is used in a crime the paper trail is going to lead to yourself.  The resulting legal headache (even if you have done nothing wrong) is not likely worth it.