It would obviously not be popular with some, but in much the same way as ebay has a listing fee it could provide some, no doubt much needed, income for STW towers.
I'm not thinking a lot, maybe 50p per add, with perhaps wanted adds staying free so people still get to sell their kit if someone else needs it?
I could see the wanted adds degenerating in to people trying to offload their vaguely relevant stuff, but I'm sure that would be self policing by the more vocal classifieds users.
What do you think, would you be willing to cough up? I would, especially if it was a nominal amount like 50p, I think it would stop a lot of the overly optimistic adds, & make finding stuff you need at sensible prices easier.
Perhaps the adds could have a time limit?
I don't know if STW is THE secondhand market now, but since the demise of the old forum, and the ability to 'sort by topic' rather than by last post, I find it very hard to use & the constant 'bumping' means a new add only stays on the front page for a few hours.