In other places, if you have to pay something in order to access a service, it's called a fee. You can only advertise things for sale on this site if you "make a donation", it isn't voluntary, it's compulsory.
Your right.
I think we need to change the wording from "donation" to "fee" for sellers placing adverts.
That's the easy bit
Trouble is the time it takes up to moderate commercial sellers.
But aren't administration costs the reason for the fee?
A couple of years back we had one commercial seller with over 500 links on the forum.
It's very cheap to get a few Indian kids to make forum accounts and post good feedback as buyers from a company,they can use a proxy to register accounts from around the world,the only way to stop it is with hours and hours of moderation.
And how do we draw a line ?
Should we take money from one company while not letting others advertise,do we take money from Bayer ect for hive products ?
No thanks...
With respect, I think this is moving the goalposts, and changing nature of the discussion.
You originally asked about one particular seller who gave some money so they could advertise something they wanted to sell. Their advert was withdrawn, so they asked for a refund. In that instance I think it's unreasonable, and possibly against a gentleman's agreement, or an understood 'contract', for the money to be retained.
Others have a different opinion of what you should do, and in the end the decision is yours to make, and only yours.
If you are going to keep the 'for sale/wanted' forum section then you, and probably Hivemaker (as Moderator), are the only ones who can work out how to deal with what you refer to as 'commercial' sellers. You need to clearly define what you mean as 'commercial' for a start, and then work out how you can separate or distinguish them from people who sell regularly sell bees or bee-related things (including services, or courses), and who maybe also have a website where they advertise their wares.
Then you have to think about people who would like to sell something once in a blue moon because it's surplus to their requirements.
Perhaps, ultimately, it might be better to charge for an entry in the directory or auction, and only allow truly casual sellers to advertise within the main body of the forum. But that won't stop people communicating via personal messaging, but worrying about that leads to nightmares and paranoia.