Vireye, don't trade it to the gun shop. They are in many cases like used car salesmen. They do need to make a profit but, if your buying a new gun the same day they should give you a better deal on the trade-in.
Try selling your Gen 3 on your own. You are more likely to get a better price for it.
I buy guns with one thought in mind. I am never going to sell it. I may trade one but, it would have to be for another gun and a even up trade.