Many of the guns were probably junkers. But a photo accompanying the story shows a pile of guns including at least one M-1 Carbine, an AR, and maybe a stock World War II type Mauser.
I've seen post-war knock-off M-1 Carbines like the Iver Johnsons selling for $400 to $500 at gun stores and shows. If the carbine in the photo is military surplus, someone may have traded a gun worth around a thousand bucks for, at most, a two hundred dollar gift card. An AR is also worth anywhere from six hundred to over a thousand bucks in the used market.
Turning in a gun at a buyback may have given some sheeple sellers a warm fuzzy for doing what they thought was the right thing. But many of them could have sold their guns to licensed dealer for a lot more than a grocery gift card. On the other hand, getting gift cards from cops is an easy way for thieves to fence a stolen guns. Government sanctioned buybacks are usually done no-questions-asked.