Charging a fee for selling will only make the forum worse.
Whatever fee you charge will just be added on to top of the wanted price for the goods. So In effect the "buyer" will be paying the fee. Which means they are no longer getting bargains.
Driving away people that merely want to sell, will only mean the loyal members of the forum get less bargains and equipment to choose from.
The sellers will happily go tug tree, or eBay on a insertion free weekend. Which will probably then end up linked on this site anyway.
Not everyone that comes to sell is a bad unwanted person. Some come here to get rid of equipment, but then get into the content/banter/conversations and stay for good.
If they cant sell, you risk losing potential great members,
Plus, and this is the biggie, the for sale section is quiet on this forum. Of the forums attend this is the least active for sale section.
Compare it to the one I run(car forum so not a rival) this is majorlydead.
We have the marketplace divided int 7sectioms so you can find what you want easily.
Yet if you put an add on at 7.30 before work, when you get home at 5.30 your ad will be on page 3 atleast.
Do we charge? Nope. You just need 250 posts before you sell. And when you've had to make 250 meaningful posts, you are part of the community, and tend not to leave.
Forum sponsors could be more active, theres plenty of threads in there sections they haven't commented on, and plenty of questions in there, they haven't answered.
Granted not every question on a forum leads to a sale, but why risk ignoring the one that might?