[DragonX2]

Yes what Falados said.
The more stands open, the less gralats it takes to rent one.
If no stands are rented at all the stands cost 2g to rent.
The more stands used, the more gralats it takes to rent one.
If 6 stands (7 is all of them) are rented, the rent for the 7th one will be 15 gralats (15 is the highest you have to pay).
So it's your choice if you like renting a stand for 15 gralats and not even making 15 gralats while selling stuff or renting for 2 gralats and making more than 2 gralats to earn more money...
If all 7 stands are rented, Markato says "All stands are rented" meaning you can't rent any if they are all taken. Hope this clears it up.