From the customer's POV:
One problem with a web based POS system like that is if you host it somewhere and your DSL goes down, yer hosed re: in-store sales. If you host it in-store, then you may need in-house tech support and a better net connection. If you have more than one store, the first problem crops up again. If you decentralize your DB, you have to worry about synchronization. What if someone buys an item online, then someone in-store picks up that item and buys it before the online order can be fulfilled (without keeping a separate online inventory)? Lots to think about.