Here is my experience (but I mainly wanted to make sure Rig-Talk still works after 10pm as I rarely see posts after 10pm - we must be getting older) LOL
They will buy stuff off you - no consignment (that I've seen)
They will buy lower than what you will expect (they have margin requirements)
You can sometimes get a good deal though
Your stuff will be worth more if you're buying something (obviously)
They may be more motivated since they are just opening
Store policies and 'wiggle room' seem to vary depending on store/manager
What I would do is plug your gear into their current used locator search and see what they are asking for your stuff now. Also get some idea of what you can get for it otherwise, so you walk with a good idea of what you are willing to accept.
If you have something new you want and GC carries it, now would be a good time to go in and negotiate.
Many GC's have gotten burned over the years because they either pay too much for used gear and/or can't resell it so if you get a store/manager that is very conservative and really only interested in selling new stuff - you may have a hard time. You should know within about 5 minutes on how your future is going to go with them
My examples:
I got my Mesa Rectoverb RII head at GC for $490
I got my Peavey Vypyr Tube 60 for $315
and I've sold some stuff to them as mine seemed willing to wheel and deal - but have not been in awhile.