How can you sell stuff fast?

Just click on the sell arena and sell the item, be careful some of them will sell 0 gold.

That depends on what you're trying to sell. As Annie mentioned you can sell most items back to the site for a small amount of gold. If you're trying to sell quickly to other players it's going to depend heavily on the item/pet and the price. Lowering the price will always make a sale more likely.

there is no garentee that what you are selling will be gone fast when it come to players bought items/pets, unless it's somthing extreamly rare or in demand that may not happen for weaks or even months. In the case you are selling a pet, i usualy advise to simply releass the cheeps pets when they reach 42 days of age, it increass your egg spawn in the LG and those have a high chance to be tagged witch can increass their value in the end. ^-^

If you nead gold there are others, faster means avalable for you at your level while you wait for someone to buy your pets/items. This guide shouldd help you with that.