I do recommend you to use both of these precious items but also caution you to make wise decisions. Both of them can be used in the Paimon's bargains shop to exchange for rare rewards and are in a limited supply. You can obtain these from Wishes, and not every player can afford to make wishes occasionally. Here's what I do with these items. For the Masterless Stardust, I exchange it for free fates from the shop and let the remaining dust sit. For the Masterless Starglitter, if there's a character I don't have, I purchase that character or get an extra constellation for an existing character. I don't recommend you to use Masterless Starglitter on weapons because you can obtain them from other banners and clear the content easily without getting the featured weapons.