The law says you can legally sell a firearm to another individual. FTF requires no FFL.
The law says that you can legally build a firearm, provided it is not intended for sale.
You need a certain license for making firearms for sale. Class-2 I want to say, can't be sure. There's different licenses for different building endeavors. I think you can even build machine guns as post-'86 dealer samples or LEO only sales, but forget what classification of building license you need.
Nothing says specifically that you cannot sell a firearm. Based on the above statements, you should be OK.
Now, if you built 20 of them and put 19 of them on gunbroker, that wouldn't be cool. Again, its the whole "engaged in the business of selling or making firearms" verbage.
Check the ATF site for more info. It will be best to call a local ATF agent asking for clarification. If you can articulate it to pose a yes/no question, you can have something in writing that says you're OK, in case anyone ever asks.
OTOH, if you buy an FFL receiver such as Vulcan, OOW, Global, then you have a full-fledged firearm and can sell that at any time. Just like an AR-15 lower receiver. Or you can add the other components, make a "complete" rifle, and sell it.
Again, if you "engage in the business" of making - OR SELLING - firearms then you gotta get FFL licensed for it, but if you only buy/sell rifles as an individual, you can do so.
There are limits - I want to say my friend who did a lot of buying/selling a few years ago read in State law that if an individual sells 10 or more rifles in a year they need an FFL, but 9 requires no FFL. That's in NC, and word of mouth from a friend who looked it up.
Call ATF, ask for clarification. You are PROBABLY in the clear and OK to do so... this all makes several assumptions.
I should add, that IF you do sell something, make a paper trail to CYA - have the buyer sign a piece of paper saying that he bought the gun from you, saying he can legally buy it, etc. That way if the police ever come knocking, asking why you sold a gun to a terrorist, you can at least point them down to the next person to whom you sold it. FWIW