Thanks folks. My original thinking was that I wanted to have a community where everyone felt welcome, without a 'rank' structure like you find on Talkbass with tiered membership. The concept being that more members = a better marketplace, which is fee paying to use. The more members use the forum (for free) = bigger audience + more people paying to list.
I say all this but in actual fact the whole model has been incredibly stable and fruitful since it began so I don't necessarily see any need to change things hugely, but always interested in opinions. I still feel, hand on heart, that the marketplace here has unique benefits worthy of a tightly knit community for a reasonable fee which increases the quality of listings and members posting to a degree making it 'safer' than other, free sites.