What's the difference between a site shop and a user shop?

A site shop is one that is stocked and maintained by the Neopets site. These shops will have a shopkeeper with whom you can haggle to lower the prices of items you wish to purchase. Site shops are automatically restocked several times an hour. User shops are maintained and restocked by Neopets users. Only items selling for less than 999,999 NP can be sold in user shops.