There is an avarage price the item is selling for, if you are looking for an item in the Shop Search that is currently selling in a shop you can see the avarage sale price. If you go to the shop then it will be a different one. The price has a range of 5% up and down (or something like this) in the Main Shops.

What I tried to explain to you that each item has a average price it's sold in the Main Shops. That's only the average price, not the REAL one, it always goes up or down some sP. That's normal. And you can still buy it for less then the price, that's why there is the option to set your own price within a limit.