Don’t worry, the calibre of people that line up to cash in guns for chump-change aren’t smart enough to have acquired weapons of any value.
Au contriare, people likely to sell guns to a “buy-back” have higher odds of having inherited something valuable, the prior owner (spouse, parent) having paid a great price for it (money or personal risk), then having died the ignorant recipient have no idea what it is or what to do with it and “just want it out of the house”.
An ugly old Sten Gun brought home from WWII (and duly registered) doesn’t look like much, but (thanks to US Code 922(o)) is irreplaceable and could easily fetch a five-digit price. Engraved/inlaid work can also have high price and significant artistic value. These things do happen.
Even most of the mundane crap turned in have pawn value exceeding the “no questions asked” price of a “buy-back”. Then there’s the point that the true crap turned in in is worth nowhere near as much (rusted-beyond-repair frames, missing major parts, etc.) as taxpayer money wasted thereon - but others have addressed that.