Won's Shop is the small area inside Zack's House that is where Won sells his products. The only products that he sells that you cannot get from him here are the vase, and the special types of apples (he comes to your door to sell you those.)
Won's shop overall is overpriced, but offers unique items not featured elsewhere. This shop appears in Friends and More Friends of Mineral Town. However, if you befriend Won in the female version, he will allow you to sell items to him. This means you'll be able to sell items that you're not able to put in your shipping box, such as flowers.
Befriending him in the female version also grants you access to play an apple game with him. Playing this game is a requirement for marriage if you are courting Won.