There's a really broad range of surfboards available these days thanks to the big steps in manufacturing and the types of surfboard businesses that have evolved in their wake.
This means it has become easy to buy a good surfboard for a lot less and with a much higher degree of certainty than you could 20 years ago.
Shop around even a little bit and you can get hold of two DHDs for less than $500 each.
Shop around more and you can do considerably better than that.
In addition, thanks to the demographic expansions of recent years, there is a much wider range of surfers these days. This very much limits the market for any particular second hand board. For instance, I have a lot of very fantastic used boards right now, but they are useless for any surfer under 15, and most surfers over 15, and certainly all surfers over 50. Legion is clearly suffering from the same issue.
So: the combination of ease of purchase of new boards, and sheer demographics, has hamstrung the second hand board market.