So.... Answering your question as to why auction had a fee and forum doesn't....

Selling through auction is selling through a median (middle man, you may call it). And the middle man has an operating cost. But with the cost it comes some benefits, like it operates without your presence and effort (you list it then wait for it to sell), it's also visible to all players online.

Where else for forum, more more effort has to be put in on the seller's side, and there's no middle man. So with forum selling, the seller needs to put in time to create posts, check frequently to see if anyone is interested (or be online for messages in game). On top of that, you need to also schedule meeting time suitable for both buyer and seller, and you also have limited number of viewers (forumers only)

Hope that explains why there's a fee for auction but none for town sells/forum trades