I am a newbie and will happily become a supporter, once I've found my feet in this new world.
An idea may be to ask a say £5 admin fee payable from the buyer if a purchase is to be made... Supporters can be exempted from this.
Advantage: Those newbies signing up for a one off make their contributions... Those that choose to stay will then find it in their interest to pay the supporter dues rather than a fiver per purchase...
I'm really grateful to be a part of this community and am in awe of the work you do... Keep it up and dont take any crap...