If you tried to sell your current bike privately it might take awhile and in the meantime the bike at the shop is just sitting and waiting for you. You might get more selling privately, but you forego the instantaneous trade in opportunity if you do it at the bike shop. I agree with chicago above in that, if you felt you owed an explanation, you could phrase it like she said. If your favorite place can't do anything for you go to the other LBS. Maybe you could buy all your new accessories, etc. at your favorite place so they know you're still a loyal customer.
Let us know what you decide.