Why do dogs bark at postmen and sellers the most?

Dogs can smell fear.

Because for them these are regular intruders.

cause they are wearing that silly outfit and mainly cause they have...in born hostility against them.

Besides the obvious protecting their territory issue, a big part of it is that when they bark at the mailman, he goes away. They don't realize that he would have left anyway, but essentially, they train themselves to keep barking. Have you ever noticed the neighborhood dog that barks at everyone when they walk by? Same thing...you keep going, so the barking must be working, as far as the dog is concerned.

Probably because they're strangers intruding on their territory. Dogs interpret that as a threat.

THESE PEOPLE HAVE THE MOST SCENTS ON THEM, OTHER DOGS,CATS PEOPLE, ETC. PLUS MOST SALESMEN ARE SHYSTERS.

Dogs respond to our body language. You are queuing the dog to bark by how you react to different people at the door. Are you particularly animated when postmen and sellers come to the door? Next time try to be casual about it and your dogs will not bark so much.

regular intruders.

they don't recognize the person and feel they are a hazard to them