Firstly, I also think that £20 is reasonable. I've used the market place equally for both buying and selling, and while the jury's out on whether it's saved or cost me money (I kept basses a lot longer before finding this site!) it's definitely been a pleasant process overall.
However, a lot of people initially come to this site to sell and a proportion stay on to take part in the forum. I can understand that BC must have running costs but the forum has for years (I've been around through Basworld and Basstalk) been run for the benefit and development of all things bass and I'm concerned that charging for sales is pushing this down a business route and may put put off some individuals from becoming part of this great forum. Sorry to be negative - its my opinion only but hopefully as a long standing member it will be respected for what it is.