A commercially made receiver that was purchased through an FFl is like any other firearm, you can buy or sell it as a private sale to anyone within your own state. Out of state shipment would require it go through an FFL on the receiving end. That's Fed laws, some states have restrictions on private sales and require you go through an FFL.
Homemade receivers made by unlicensed individuals are a can of worms. Technically, you could sell it just like any other if you were to stamp the required manufacturers info onto it (name, city, state, caliber, and serial#) but if you make a receiver for the purpose of selling it you must be licensed. The only reason an unlicensed individual can make a firearm or receiver is for personal use. With an assembled rifle, it can be used and then sold and most people would accept you built it for your own use then needed cash, didn't want it anymore, etc. An unused receiver OTOH is a tougher sell on the personal use issue. There are also liabilty issues, there is a reason firearm manufacturers have multi-million dollar insurance policies. Most dealers are not up on the homemade receiver regs and issues, so they just refuse to deal with them at all.