In the US at least, there are three classes of people who are willing to sell guns cheap at a buy back, and this is why the concept has never taken off, along of course with the cost, which isn't effective at buying votes:
Criminals wishing to dispose of weapons used in crime.
Savvy types who sell guns that are worth less.
The ignorant/those who don't want to be bothered to easily dispose of guns that they've inherited or whatever.
The savvy types will work the line to buy guns from the latter that are worth significantly more than the price.
