Are there mods that mark unique items in skyrim?

Legacy of the Dragonborn is a mod that allows you to fill an entire museum with all the unique items from Skyrim and more (3500+). This mod has compatibility patches with a lot of other mods in case you use those to incorporate their unique items as well.

The best feature i.m.o. of this mod is that it also helps you find unique items from time to time.

It does not 'mark' the items, but if you go to the museum and stash your items after exploring an area you can at least filter all the museum items through the drop-off chest in the working area or the various shipment crates throughout Skyrim. After that you can sell the rest yourself or use the selling cart feature of the museum.

Note that you do need to start a new game for it to work. Also, read the manual and various disclaimers on the main page of the mod so you don't run into any surprises.

Legacy of the Dragonborn is also available for oldrim.

https://www.nexusmods.com/skyrimspecialedition/mods/17824

I would greatly recommend this.

It's not as good as Mixxiphoid answer. However I need only to check my keepsake items once. After that it's good.