In fairness, there was an improvement a couple of years ago, when the second-hand basses started to be categorised by number of strings and price range.
There's some small things - like the fact that the "Bass guitars | Amplification | Effects | Accessories" menu line is still shown when you go into any of the categories except "Bass guitars", and it also disappears if you drill down to a manufacturer.
Having a consistent header and menu structure (banner header, the four sales categories, and a dropdown on each of those categories containing what the sidebar does now) means that the page content could be unchanged but the way of getting to it would be improved. I suspect Mark would have to change website generator though, as he's using the final version of iWeb which came out in 2011.