Would restricting their sale make the anti-gun crowd happier? Yes. It's not a matter of prevention,
it's a matter of perception.
How do you really restrict the sale of existing guns? If you simply
say they are not legally transferable, a lot of people would sell them
under the table. Most people are not going to just throw away several
hundred dollars for an AK/AR or well over a thousand for my M1A.
Actually there are AR platforms that are well over a thousand too.
A basic "starter kit" AR is $400-500. You have to assemble it (the lower). A good assembled AR is about $1000.
Building one is easy. Once you've done it, you can build a lower in ~20 minutes. The upper, if you have the couple of tools required, shouldn't take more than that.