One reason for the general failure of gun buybacks in the US is two responses from the gun savvy:
1) Go back a bit from the front of the line, and if you see a valuable weapon held by a noob (vs. a likely crime weapon), buy it on the spot for a lot more than what's being offered.
2) Gather up clunker weapons with values much less than the $100 or whatever they're offering and make out like a bandit.
http://www.foxnews.com/politics/2015/01/24/firearms-enthusia...
As far as the feds are concerned, as long as you're not "in the business", you're OK, and these buybacks tend to be rare and one offs. In practice and from their viewpoint, the first case of valuable guns is OK because there don't tend to be too many of them and there can be many different people doing this for any one buyback, in the latter case it tends to be FFLs who play that game since they can easily have a collection of junker guns they've bought but that didn't sell.