>>Note : I have not seen any used Joe Mullins models up for sale. That tells me the people that bought these must like them.<<

Or that not many of them have sold yet. Or, that since its a pretty new offering not that many of them have sold yet.
GT is really into offering a lot of different models right now and I expect it's going to take some time for any of them to start making their way onto the used market, this one included.

Bobby

This model hasn't been out anywhere near long enough for many to have sold, let alone to have made it into the used market. People would have to have got rid of theirs almost immediately.

And as large the Banjo Hangout membership is, it's just a fraction of the banjo buying world. 

Plus, I distinctly remember from the launch of the OB-3 Twanger when Banjo Ben's General Store had it as an exclusive for maybe its first year, it arrived in batches of 36.

As to sound, we all know that setup is everything. Choices and adjustments in strings, head, bridge, tailpiece, action, relief and who knows what else can lead to the same banjo, not just different banjos of the same model, sounding great or terrible or any point in-between. Three years ago, I played a great sounding OB-150 at a music shop in Paris. A few months ago, at my neighborhood music shop, I played two unimpressive new Gold Tones of their higher up model. I think they were the Standard and Grandee. I chalk that up to setup. 

If you've heard a video in which the Mullins banjo sounds great, then accept that as demonstrating what the instrument is capable of. If the same model in the Banjo Warehouse video sounds worse, then accept that as the banjo not being setup to deliver all it's capable of. Or it's the player. Or the recording. Or the room in which the recording was made.

A banjo is never going to sound better than it can sound. But it can easily sound worse.

Regarding Joe Mullins, remember he plays HARD and LOUD.