Sets, no. Don't just outright sell a set because it's set bonus may not be great.

But items, absolutely. Depending on your progression you will have increasingly strict filter requirements. For an early game player, you can probably use the following rules to decide what to sell:

1. Everything under 4*, and everything with flat stats on the bottom row - HP/DEF/ATK. To be clear, this is not the same as HP%/DEF%/ATK%, which you want to keep. 
2. All 4* rare items. All 4* epic items with flat HP/DEF/ATK substats. All 4* legendary items with two of flat HP/DEF/ATK substats.
3. All 5* rare items with flat HP/DEF/ATK substats . All 5* epic items with two of flat HP/DEF/ATK substats. All 5* legendary items with three of flat HP/DEF/ATK substats.
4. All 6* epic items with three flat HP/DEF/ATK substats.
5. For the items that remain, roll them to level 8 during artifact enhancement, and sell any that hit flat HP/DEF/ATK substat.