On your materials list under Sale Items your materials will either show a white tag box, indicating that there are no tags to this materials, or it will show a blue tag box which indicates that the material is tagged.
To view your materials list and see whether your sale item material is tagged then navigate to Sale Items and select Materials.
As you can see below, the blue box indicates the material as tagged (tags listed on the right) and the white box indicates the material is not tagged.
If you wish to tag a material that is not tagged already then select the white tag box.
Select a Sale Item Tag and then when happy hit Save.
As you can see below the tag has now been saved. Also, there is now a blue tag box indicating the material is now tagged.