Is there a name for those who take hitchhikers in their cars? (compared with buyer and seller).
From the point of view of the hitchhiker, you could call the person "a ride" (this is an example of metonymy). It is rather casual.
Yeah, my first ride had this big moustache and spent the whole time talking about his pets.
You would not say "I am a ride". You would say "I pick up hitchhikers". In more formal contexts "people who pick up hitchhikers" is the simple and plain way to express this.