I'm somewhat hesitant to ever say out loud that Guitar Center has some *phenomenal* used gear prices in certain cases. I don't know if they realized that, and are as a result scaling back their used inventory - but I've never known them to *not* accept an item I tried to get them to buy either.
One potential reason for the lesser volume is that they are actually very bad at updating their online inventory of used items - they might just be behind. I've seen gear they bought from me show up two months later in the store's inventory - in some jurisdictions they have to wait a certain amount of time, but they're also just lazy. They also tend to shuffle their used inventory from store to store as-needed, I had a used instrument I sold here in Orange County ultimately get shipped to Texas to be part of that store's inventory.
I actually have a 'bad habit' of buying cheap gear on Craigslist etc. , if it's cheap enough I figure I can double my money by selling it to Guitar Center (and only Guitar Center.)* Even if I could get more selling it on Reverb/eBay, I hate dealing with 'human' buyers, having to ship, etc. - Which leads to my next caveat, Guitar Center barely inspects the instruments they take so it's not inconceivable that they will sell a lemon without realizing it, or being knowledgable to realize it.
But if you're willing to gamble and bank of having to do some possible setup or repair work, you can get some amazing gear for less than other venues typically sell it for. In fact- it pays to look out for guitars they're selling that they don't realize have upgraded pickups etc. , or that are mislabeled (mistaking a US G&L for import etc.) So I hope Guitar Center continues the status quo with their used gear - but I think going from store to store, the most I've ever seen is a couple used G&Ls in stock at a time.
(*Another aside - they're very inconsistent with how much they give you for your stuff. I once bought a $100 Peavey 4x12" from Guitar Center in Lawndale for my own use, but was not too fond of the tone so sold it to a Guitar Center in Orange County in the exact same condition a couple months later. They gave me over $200 for it

which was about $50 more than the average selling price of the same cab on Reverb. Can't explain it, but they've often been -very- fair to me when buying my gear.)