Board Thread:Questions and Answers/@comment-176.63.9.235-20160115125358/@comment-25968376-20160115171714

I'd say the useless ones are lower-grade versions of treasures with the same effect (e.g. 3% extra XP vs 10%). But if you're a collector like me, I wouldn't sell them unless I had duplicates.

You only get a few coins and MP for selling. It's only worth it to sell them if you're early in the game and need space for more.