How do you use heart scales in Pokemon diamond?

you can sell them in stores but you can trade them in to teach your Pokemon new moves in pastoria.

Answer

If your desperate and you taught one of your Pokemon an HM move that you don't want there any more you could first get that HM move deleted by the Move Deleter in Canalave City. And if you have a Heart Scale you could reteach that Pokemon it's move that you lost when you had taught it an HM move. I think that guy is in Pastoria but I forget.