First off, don't take "buyback" too literally. These people are operating in a world where terms mean what they want them to mean, no matter the proper definitions. They are buying guns to get them "back" off the street. So, to them, its a "buyback".
Second point, its NOT a "confiscation". It's a totally voluntary, nothing is being taken without or with low compensation. The guns are being "sold" by their "owners free will"
Next point, the one's I've heard of somehow operate as some kind of amnesty (no idea of the legality), but guns are accepted with no checks on the sellers, or, it appears, the guns themselves. Seems to be a good way for a criminal to get rid of a stolen gun /gun used in a crime, without any risk to themselves, and with a touch of profit, to boot!
There was one in my area a few years ago,buying guns with a $100 or $150 gift card, and as you might expect, nearly all that were "bought" were non-working junk. There was one guy, who ticked off the program managers, though. He stood outside (on public property-the sidewalk) and talked to people before they went in to "sell" their gun(s).
If he found someone who had something decent, he offered them a decent (if lowball) price for it, which was always more than they would get from the "buyback" program. And, what he was doing was entirely legal. All sales were conducted in accordance with the law, when transacted.
There hasn't been another of those programs in my area, since....