This translation depends on the context. "Der Verkäufer" could refer to the seller as in a contract, or to the salesperson in a shopping scenario as in a Kaufhaus, but that is not necessarily a dealer. "Der Verkäufer" really doesn't fit "Der Händler" either. "Der Händler" refers more to a merchant as in a wholesale situation. Handel is trade. i.e. Obst und Gemuese oder Wein Händler. That is where "dealer" sounds more likely as a translation. Now what if this is a casino and we are referring to a Kartenhändler? Which by the way I have never heard used. I have only heard that kind of dealer in German speech referred to as a croupier. There isn't a context here so the translation is imprecise.
One cannot tell since the test item is generic. It would need to be proceeded by something specific such as "drug" (Drogen) dealer (Händler) vs. i.e. Schrotthändler. But "sinister" is also open to interpretation.