Hiring a consignment merchant requires that you have a side porch add-on. After that, you just need to buy the merchant from the deed merchant in the central market area. Once you've purchased the merchant, drop the consignment merchant item in your inventory anywhere on the outside of the house. The merchant will appear immediately on the side porch and will be ready to get to work selling your items. Right-click on the merchant to open his inventory. Now, drop each item into the merchant storage window.
As you place each item into the merchants inventory you will be prompted to set the price for each item. Once you set the price, click on accept and the item will now be ready for sale. Other players will only be able to buy from the merchant. Once an item has been bought, you can return to the merchant and press on the withdraw button to collect your earnings.
To remove items from the merchant, open the inventory and click and drag the item back into your own inventory. Note that you can't drag them item over another item in your inventory to quick swap the items. You'll need to transfer all items from the merchant to an empty slot in your inventory.