I think the Mog Shop could be more on a personal level of buying goods. What i mean is that they can take the Disgaea route and make it where if you bought items from your shop, you'd level it up, where you can buy better equipments and stuff.

Along with that, you'd need to sell certain materials to your mog/retainer in order for them to create new ones for you to buy. So say you wanted them to be able to sell Brass Lances, you'd have to have leveled your shop to level 17-20 and have to have sold enough Brass Ingots and Copper Lances in order for them to sell Brass Lances.

However, that is not to say they would sell them at a reasonable price, crafting one yourself would be far less costly, but not everyone likes to craft, so this would make for a great alternative.