You can glorify the role of the middleman all you like, but at the end of the day they’re nothing more than parasites on that very simple equation.
Vocabulary to help: Opportunity Cost.
The middleman pays the opportunity cost, and assumes the associated risks of owning an item. These are real things to the market place, and they can and do benefit the person who crafted the item, as that crafter can now go out, buy more mats and make something else. Sure, that middleman may make a 20%, 50%, or 2000% profit when that item sells in a month. Sure, its more work for the crafter. Sure, its a smaller margin, but the crafter has not paid that cost. The crafter can roll over that one gold at 30% a day, and keep rolling it over for (in an idealized economy) a final value of 2500 gold after a month. Oh woe is the flipper, who took spent that one gold and made a meager 20 gold when the item finally sold at the end of a month.