We will typically the kit version of a certain vape, or we will carry the vape and tank separately. We have found that when we carry the mod and tank separately but also carry the kit, either the kit or the individual items won't sell very well.
Kits are often a great deal, typically costing at least 20% less than if you were to buy the mod and tank separately. Take the Smok Stick V8 Kit for example- the Big Baby Beast tank that comes with the kit costs around $35 on its own, and the Stick V8 Kit costs about $40. That means you're essentially getting the stick battery that comes with the kit for about $5.
Keep this in mind if you discover that we only carry the kit version of the mod you want, as whether or not you need a new tank, you're not paying a whole lot more to get the mod with the tank compared to getting only the mod.