The moment you tell people "for this to work you have to go buy something else separately," you lose a huge chunk of potential sales. People don't want to do the work of figuring out what specific model of something else they need, where they have to go to get it, etc. Even if you put the something else on the shelf right next to your thing, the added cognitive load will turn lots of people off.

Build the other thing in to your thing, and all those problems go away.