I know little about gun buybacks, but my basic understanding is this: they are government sponsored programs to "get guns off the street" (because, you know, the guns can go astray or something) and offer a modest cash amount and (in most places, it seems) no questions asked. Someone correct me if i'm wrong here.
Going off this premise, it seems there are three reasons people would turn in guns:
1) Stolen guns, looking to make a quick buck
2) Broken/Defective guns. From an old shotty with a busted barrel to a Lorcin which never quite worked right, someone's just looking to get a few dollars.
3) "Do-Gooders" who honestly believe that guns are evil, they cause crime, and want them all melted down.
So, if you did this (standing nearby and attempting to buy them) here's what you face:
1) Stolen Property - I don't think I have to elaborate
2) Junk Guns. Unless you're looking for a parts gun for your Hi-Point you just can't bear to part with, these you don't want.
3) These people, because of their feelings, are unlikely to want to sell you their gun anyway - they want to give them to the government....because they're the only ones who need them
Ultimately, it doesn't seem like the time spent (and possible hassle from the cops) would be worth the small likelihood of payoff.
ETA: After reading the thread more closely, it seems I forgot another class:
4) The widow/widower/heir who gets or has a gun that a relative is no longer shooting.
These would most likely be where the deals would be found...but I still don't think these would come along often enough to make it worth it.