Obama's royal decree is to also add another 200+ ATF agents to investigate and prosecute this stuff.
I am pretty sure that a lot of them will be sitting in front of computer screens, looking for these types of forums, and data mining for "violations"...
Also consider that if they decide to make an example of someone who trades or sells 5 guns per month, claiming that they are a "dealer", there might also be IRS hassles to deal with in association with that... I mean, if you are deemed to be "running a business", then you need to be filing tax forms for that business, no?
The wording is vague enough that this could open all sorts of cans of worms...
If Congress does not add the money for 200 more agents, he can't even do this. I have no faith in Congress, but they do still control the purse strings.