I found one for sale and I was wondering if it would be against NFA rules.

If that's all it is, then yes. According to NFA rules all the way back to the GCA of 34 (I think), any and all rifles must have a minimum barrel length of 16 or 16.25 inches. This is why all of the AMD-65 kits had to have a permanent extension added, or they had to be made into legal SBRs. If you want the short barrel, you need to either pay the tax stamp (if legal in your area) and install it or add about 5.5" or more of barrel extension, like a really long flash suppressor or something else that is permanently attached.  

That is what we call a short barreled rifle.
Is this an AMD65 kit?

It isn't an AMD-65 kit it is just a 10.5in barrel. I think it might require a form1 from the BATF.  

Yep, gotta register that thing as an SBR, unless you have a permanently attached muzzle device.  MJD

You can use it if you make a pistol from a virgin reciever that has never been assembled as a rifle.  Keep in mind that the short barrel is only the beginning of the project.  You will also need a custom gas piston, modified gas tube and a couple of other things to make it work.  Other than that you will need to permanently attach a barrel extension to bring the length to the required 16"

Thanks for the advice, I'll hold off on that project for now.