If you want things to have value out of game you don't want to sell them in a store at fixed prices. You want to sell them through player to player trading (you take a cut of the trade of course) and you have to spawn them inside the game world somehow, with varying rarity. At least that's how it worked in Team Fortress. I sold my Vintage Merryweather for over £50 in the end. This is only good for the game world as long as the items you can buy aren't game changing. Pay To Win would be the kiss of death I think.
So eventually what you need is player trading, out of game through some forum API or other, of in-game items.
So eventually what you need is player trading, out of game through some forum API or other, of in-game items.