Sell Shops are where you are able to sell off items for money, in order to level and buy various items on the server. The items that will be the most valuable, are going to be crops. These are farm-able items such as Wheat, Beetroot, Potato, Carrot, Cactus, Nether Wart, etc. For a more comprehensive list and crop pricing info, check out /prices in game.
New Sell Shops are unlocked by progressing through the levels. In regular leveling (not Prestige), these shops are unlocked every 3 levels. For example, you unlock your 2nd Sell Shop at level 12. Your 3rd Sell Shop will then be unlocked at level 24 (increment by 4, remember?). The 3rd Sell Shop is going to have higher pricing than the 2nd Sell Shop, and you should always use the highest Sell Shop available in order to get the most money. Use /sellall to accomplish this.
Sell Shops within the Prestige leveling system are different. There are 6 of them, and they are unlocked using Prestige Credits.