First press action while facing the counter with no items underneath your character or on either side of the counter to clear any price you’ve already set.
The first item you drop is your price so in this case gold. Drop your gold in the amount you want to set the price to then press action and pick up the gold again. The price is now set, if you want to change the quantity of price later you can simply type /price # while facing that counter.
Next you drop the item you want to sell, so stone. Drop your stone in stacks of 250 on top of each other then press action. From that point forward you do not need to press action again to set a new sell item, you simply drop the item or stack at the counter and it will sell for whatever price you have set.
Counters with nothing set on them can be used by other players to send items over to the other side of the counter.