In 2006, there was more than one disklavier option available for the GC1. One of those was the Mark IV which was actually more advanced than the later released E3 system. The current Enspire Pro is more on the level with the mechanical accuracy of the Mark IV, but with additional new improvements, tools, better connectivity, and a nicer interface.
Just out of curiosity: how accurately do circa 2006 Disklaviers replay, say, your playing? I mean, if it were accurate, then it seems to me it would be a fantastic practice tool.
If it is the Mark IV
If it is the older Mark III system, it may be partially upgradeable to the DKC850 (if still available) that more or less turned the Mark III into the E3. It is not upgradeable beyond that.
Whether it is a Mark III or Mark IV, you should expect to have some service done to still enjoy the player system. On a Mark IV, for example, you can add a wifi extender and then run Yamaha's app to control many of the entertainment functions with your phone or tablet.
jshelton...9 years ago, we were still deep in recession. For someone to buy a GC1 in 2006 (presumably for ~$15k then) and turn around 4 years later to sell for $5k is not in anyway typical. Congrats on your steal, but its not good context for the normal market.
In our local market, a new GC1 w/player sells for close to $30k. Applying typical depreciation for this situation gives you different numbers. While we don't sell many used GC1's, all the ones we have sold (without disklavier systems) from the mid-2000's, sold for $11k or more.
Can't inspect or confirm the condition of the one in the ad, but if it was very good to excellent, it would be below retail for our area.