A dragon dealer is presumably a wizard or witch who illegally deals in dragons. Rubeus Hagrid mentioned this profession in 1992 when Harry questioned him about where he got his dragon egg. Based on this conversation Harry suspected that either Snape or Voldemort had disguised themselves as the dragon dealer that had given Hagrid a Norwegian Ridgeback egg in order to try and obtain information from Hagrid about how to get past Fluffy. Later, it was revealed that the cloaked figure in question was Quirinus Quirrell under Voldemort's services.