Well consider the futures markets: Buying and selling nothings that will hopefully become somethings. Oil works that way, as do any number of goods that require time to grow or mature or process before they hit the market.

Then consider: movie theatres sell the idea of a distraction or entertainment. We ride roller-coasters for a completely ephemeral high. Some of our most expensive products are ideas, without physical form.

What the gold farmers are selling is ideas, and why not?