Blocks- Commonly used building blocks and natural blocks.

Resources- Raw minerals and valuable minecraft items.

Farming- Farmable blocks like crop seeds and saplings.

Food- Food items.

Mob Drops- The things that Mobs Drop.

Spawners- Several common spawners type.

Spawn Eggs- A few mob eggs of mobs that are hard to find or to help get you started.

Special Items- This is where /shop gets interesting! The Special Items section includes Chunk Hoppers, Safari Nets, Dragon Eggs, Invisible Item Frames, and Towny Chunks. 

Auction House is where you can list items for sale to other players! Simply hold the item in your hand and do /ah sell [price]. Access the auction house by doing /ah

Player Warps are how most people set up shops but they can also be used to showcase builds or any number of other things!
You can access the player warp gui with /pw. Other player warp-related commands include /pw create (name) to create a new player warp (this costs 50,000). /pw reset (name) to change the location of your player warp. There are several other player warp commands related to customizing your warp once you've set it. These can be seen in-game by doing /pw set and looking through the auto-fill options.

People often set Chest Shops at their Player Warps in order to sell large amounts of items to players!
See The image for how to set up your chest shops!

Leave the top line blank, put how many items per click you'd like to sell or buy on the second line, and the price on the third line. 
If the name of an item is too long to fit on the sign you can simply put a ? on the bottom line then click the sign with the item or type /iteminfo with the item in your hand and type it out!