You don't don't like the fact that the dealer is willing to give you more for your item if you buy another than if they just buy it from you outright. It isn't shady or complicated. It is actually very much up front and simple.
It is a common practice that happens across a myriad of industries/businesses where sellers are also buyers. Your will find it with guns, cars, boats, planes, industrial equipment, furniture, farm equipment, jewelry, and so on.
For me, there are two prices on everything ... FMV and what I want for it.
If I'm ready to move onto something else, I'll price it accordingly.
If I have something you want to buy and I'm still "attached", I'll price it accordingly.
I won't advertise two prices.
Ah, but you DO have two prices depending on your emotional attachment. The dealers two offers to buy are not based on emotional attachments, but in trying to make deals and in doing so to make money. They would much rather pay a little more for your gun and sell you something and make some profit now and then profit later on selling the gun you sold them, than to give you a bunch of money and have you walk away, no direct profit being made from the transaction until sometime later when they resell your gun. A bird in the the hand...
Think of the higher offering for trade-in as the incentive to buy. That is all it is. It isn't shady in the least.