You are wrong with your assumption Kaufmann means someone who works in/for a grocery store. It's the common name for a grocery store.
But Kaufmann is also the name of a skilled trade, and there are many many specialized apprenticeships of that kind, all with that word in it. There's also the eingetragener Kaufmann, which is a law term for a single-person trade firm with some special properties. And of course, there's also the Kauffrau.
A company – more than one person – in contrary is a Gesellschaft. That's the law term but it's also used in common language. A Handelsgesellschaft is a company which trades goods. Same with Vertriebsgesellschaft. If it sells to the general public, it becomes a Einzelhandelsgesellschaft. (Same with the Einzelhandelskaufmann.)
There's no special name for a company that sells to companies only (or mostly). Großhandel exists, but that's more a class than a name. No one writes Großhandelsgesellschaft.
You can also simply write Händler for someone who trades, a person or a company:
Ihr Händler für Point-of-Sale-Systeme
Ihr Großhändler für Point-of-Sale-Systeme
You may want to use Ausstatter for someone who sells appliances.
Ihr Ausstatter für Point-of-Sale-Systeme
Your supplier for point-of-sale systems.