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.