We'd hope that anyone coming here to sell items around the £50-100 mark are probably selling other items too, or at least members who have signed up for more than to sell these small value items. In that case, a donation from them for the use of the forum in general is probably a reasonable ask.
We can always adjust the prices and so forth at a later stage, but to begin with we can't customise it any more because we can't afford to do so, yet.
Cheers