Sorry, but you were at least partially wrong.
If you think a dealer doesn't offer enough on used guns, don't sell your guns there. Tell all your friends not to sell their guns there. Get on the internet and tell anyone that will listen not to sell their guns there. Heck, I'll beat you to it:
You will not get a good price from any gun dealer for your used gun, any more than you will get a good price if you sell your car for cash at a used car dealer. People sell their guns at gun dealers because it's quick and easy, not because they offer top dollar.
However, when you are in a gun dealer's shop, under the roof he pays for every month, what he offers a seller is between him and the seller. It's none of your business. The dealer doesn't run the shop so you can hang out there and cherry pick his used gun deals. If you want to do that, open your own shop and see how easy it is.
The saving grace is that at least it sounds like the seller turned the offer down first. That lets you off the hook a bit. If you had said something during the negotiations, you would have been way out of line.