As things are set out for sale by other players of the community, they are rezzed on the ground (or a shelf etc) and they eventually sell, however, many people forget or simply do not know how things rezzed for sale works.
To buy an item, rightclick the item, select buy and click "buy now", after you have bought the item then remember to pick it up to your inventory by right-click and "Take", congratulations! you now have your bought item.
Once the bought item is in your inventory, you will have to wear it (double-click item in inventory) and then left-click on it and use the menu to consume the pet powder or potion. (for powders please have your pet attached already)
Potions are consumed right away and goes directly to your rod, while pet powders required you to either select GH (Gold Hunt) or FH (Fish Hunt) pet, this is due to the pet powders being the same for both pet types.