This is good news Dave.
The wheels of justice turn slowly.... just remember that. Be patient.... but be persistent as well. Yep.... I know you want the gear back and I know you want to see the perps in a federal super max with bread and water rations, but things take time.
I'll bet GC does everything they need to and more to be accommodating to you. They do not want the reputation getting around that they buy stolen gear. So be slow to use online retaliatory measures, in other words, keep them as friends on your team and don't burn the bridges just yet. There will be plenty of time to do that later if it is even necessary. It seems that they are interested in helping you.
Visit the store and see if there are cameras in the store. If there are, have them save that section of video of the perp coming in. Remember, only one person has to slap ID on the counter. The actual thief could have been standing right there too.... the video will tell if that's the case. Time is critical to get that video archived because 99% of all video systems these days are hard drive based and WILL overwrite the old data in a matter of time. Depending on size of the HD and how they have the system configured, it could be a few days to a few weeks or longer. So get that video off that machine. They can email it to the police as well as archive it on the machine itself.
I'm pretty sure the local PD is in touch with the PD in your town and already working together.... they will visit the seller, bring him to the station for questioning, and especially if they have video, likely get a confession and a plea deal, where the guy gets probation or 30 days in exchange for the name of the thief. Remember, the seller, if he's not the thief, is not the guy who broke in and stole the gear. He's a (patsy) guy looking to make a buck but knowing he was selling stolen gear. Not too likely he is willing to risk going to prison for a few hundred bucks. He'll be willing to give up the name. Then again, it might not have been the neighbor, and the thief is a complete beginner and actually stole it and sold it himself and gave his ID to the store. Those kinds of folks don't often think things through very well. He could have seen you at a gig, did some research and found out where you live.... heck he could have even talked with you at the gig about your gear. The guy who stole my SG from my house (the second time it was stolen) knew me and had been in my house with some friends of his who had reason to visit. Heck, he even played the guitar setting on my couch and we talked about the guitar and music. Several months later, he broke in and stole it. I got it back the next day because he was seen by a neighbor.
Just let things roll under the guidance of the (now) 2 police departments..... they are making progress and have a name. Just keep in touch and keep them focused.