you do what you can. If they don't offer the service you are looking for... what can you do?

I would talk to the LBS owner of the store you do like... tell them your situation, and say "hey, I'm not crazy about going over there, but they have this service and I need to sell/trade my current bike to help pay for my new bike... if you can think of way to make this work at your place, let me know... otherwise I will probably go their and perform this one business transaction".

Nothing wrong with that.. it's the truth right? ... and there's nothing wrong with showing up at their events with a bike you bought somewhere else? If they don't provide the service... that does not equate to you having to settle for something less than you do deserve. Support them in the ways you can.. and if you can't support them, you've done the best you can do