I have a premium and really enjoy it, but there is no denying that GB is a very polarizing game. It's got phenomenal sound and a stellar light show, as well as one of the best art packages out there. It's also a dream theme for many. That being said, it's difficult as hell, and the rules are extremely unbalanced. There is way too much emphasis on the skill shot and the playfield multipliers. On top of this, you also have the linear nature of the modes. These things cause many to find the game very frustrating and/or unsatisfying. Some of this could be fixed with tweaks to the code, but Stern has not given us any reason to believe this will happen.
It's not just GB, though. If you look at the market, there are a bunch of newer HUO games up for sale at substantial discounts compared to what they cost new. Plus, it's not just Stern. There are plenty of Spooky and JJP games up for sale as well. People can barely give their Hobbits and RZ's away. Heck, we're even starting to see some turnover on the Chicago Gaming remakes. I'm definitely sensing that the market is reaching it's saturation point. People are running out of space/money, and with all the compelling games like GOTG, Houdini, TNA and POTC coming down the pike, people simply need to sell stuff. The days of buying a NIB game and thinking you will be guaranteed to break even on it when you sell it in a year are a thing of the past.