The only possible way to enforce private sales is a national gun registry and that aint gonna happen.
Yes it will - unless the alphabet pro-gun organizations quit high-fiving everybody over the recent election and start pressuring Congress to break the FFL-only NICS link. Right now, citizens cannot perform background checks on their own. Until we can, we are screwed.
I say screwed because Bloomberg, et al will flood the various states with their money and push their version of background checks aka registration via initiatives. The Gruberized voters and those who don't give a damn about the registration aspect will vote in lockstep as they did in Washington.
Again, I pound the drum that we have to push our own people to allow any Joe citizen to make a check and cut the ground out from under the Bloomberg crowd. If we don't, we will assuredly have registration enacted state by state.