How to create in game store for multiplayer games?

I want to create an in-game store for my multiplayer game, where players will be able to buy skins with in game currency/real world currency. How should i integrate this?

Watch this first, really:

So first make prototype, create some mock-up of online shop, do not bother yet with real thing until you have game in alpha or so. One thing however, if you want multiplayer start implementing it from beginning, a lot of things work different when you want replication.

For in game shop you should try first steam integration.

THere is something about it: