Disagree with a membership scheme, it will inadvertently create a tier between users.
I think the forum shop thing that stoke mentioned is a very good idea.
I also think that the idea posted by b7unr is a good one. The forum could organise more formal meets such as track days/trips to attractions etc, and charge a ticket cost with an added margin that goes to maintaining/improving the forum. I can't see how anyone would object to that?
Whilst the current method of gaining donations (the occasional thread in the General Section) is working fine now, I think it may grow tiresome in the future, although I'm sure that this isn't your long term plan to keep doing it in the same manner as now anyway.
Do you charge the traders an annual cost to be an approved trader? Or do you take a cut of their sales via the forum? (or do they donate a cut themselves?). N.B. I'm not suggesting that this is done (don't want all you traders getting angry at me), I'm just wanting to find out the approach that the forum takes.
I'll donate shortly