Question

Where can I sell the rare berries in Black and White, like Razz Berries or Nanab Berries?

Answer 1

On the Route next to Nimbasa City, there is a maid in a trailer that wishes to buy food related items. That is where you sell them.

Answer 2

you go to route 5 then you go inside a tralier on the left and the lady ask if she can have some food item (p.s they cost 500$)