If you'd like to sell items, those options are located in your item inventory (Game tab > Items > Item Inventory)

(click to enlarge)


The bottom three buttons of the ones I circled are the ones that you can sell stuff with. The "Add To Market" button will allow you to select an item for auctioning. "Add To Usershop" lets you place items on your usershop, which is a small shop that appears on your profile, and is also searchable. "Create Personal Trade" lets you specify a particular player that you want to sell an item to (private sale).

If you'd like to buy items, you can do that by going to the Game tab > Market > Item Market or Usershop Search. The Item Market (and the Pixpet Market) is an auction house which is fairly straightforward. You can search for an item you want or simply browse what's available. When you find something you like, just click the "bid" button and you'll be taken to a page where you can place your bid.

If auctions aren't your thing, you can also check out the usershops which allow you to buy stuff instantly.

If you want to sell Pixpet eggs, it's a different story. There's a separate auction house for that (also under the Market tab), and the options to sell eggs are located under Pixpets > Pixpet Egg Handling. Very similar options to the item inventory, except there are only two ways to sell eggs, which is of course auctions ("Add To Market" button) or sell them back to the server ("Return Egg" button).

Let me know if anything I said was confusing, or if you have more questions :)