you cannot make a firearm for anyone else and the ARs you make can never be sold or transferred to any other person.
The law is pretty clear on this, you are correct that you cannot make a gun for someone else. You cannot make a gun with the intention of selling it at all.
Here is where the but comes in, you can sell it as you would a normal gun, you can even take it through a gun shop (although they will require you to serialize it and brand it) Just make sure you don't make a bunch and try to sell them out to your friends, this is very illegal without the proper licensing.