I'd say keep the old marketplace because that's what drives the whole forum. Yes you do get the odd one post wonders but I'm willing to wager alot of regular forum users came here just to buy or sell a bass, and then stuck around when they realised there's a helpful community here.
1) Charge a small fee per listing, not annual subs because that will scare the casual user away. I dare say if you do introduce a larger annual sub then some bright spark will open another site without one to fill the gap in the market you've created.
2) Introduce mandatory price and location fields when creating a thread. That's a no brainer.
3) Introduce the 36hr bump button, but no so rigid as to be unable to update your thread before 36 hours are up.
That way we can keep the discussion in the sales threads, the sense of community and self policing and keep the chat about bass flowing.