There is one of those cabs on Reverb in acceptable condition that is down to $250 and still sitting there. I could likely offer the guy $200 and pick it up this afternoon...
If there was any scheme, it was that they were trying to make it seem like it wasn't working right so they could just say "Sorry, we can't buy gear not working correctly" Did it all the time with stuff we knew was going to sit around forever killing the used gear numbers.
The reason that no one will give you any numbers over the phone, even with 8x10 color glossy photos with circles and arrows and a paragraph on the back of each one is because there is a company policy against it. There's a couple of reasons... Firstly, pictures can be manipulated and fudged and can't be verified and don't tell anything about the working condition of the gear. When you're given a range of value it can be considered an offer to buy, which only a manager can make to start, and could trigger lawsuits in some instances. Not worth doing in light of that, and when I was at GC 20+ years ago I had to fire someone for doing it.
The main reason (and it IS a bit manipulative) for not giving estimates over the phone (or email or text) is that you are more likely to accept an offer after you've lugged your gear all the way to the store. It used to be policy that you tested the gear with the customer and then put it where they could see it, but was behind a counter or just inside an office on the sales floor because the buyer has a psychological advantage in that it seems as though they've already got the gear.
In this case, though, they made a more than reasonable offer to someone who didn't take any time to do homework on the deal.
Just a touch of advice... It is not the store's responsibility to educate their customers. Be a well educated customer or be a target. Sales people are not there to help no matter what they say. They are there to make money.
FWIW, I sell stuff I'm no longer using to GC 95% of the time because, unless it's something very desirable, it's not worth the hassle of dealing with buyers. Every time I sell to GC I get what I ask for, and sometimes my price is too low and I get more. Here's the equation - you get 60% of the average price the item sells for. If it's something that is still available new, they will list the used item for 60% of the new price as long as it's in good or better condition.