There’s some great common items as well if they are levelled up sufficiently. I use Saarvin’s common hood for Wild Shot and I get 2 extra 125% shots out of the attack and will be 3 once the item hits level 13. Shevariths common hood with Gust of Wind attack is brilliant at item level 9 - pushes 2 opponents back with 125% damage plus bonus damage if someone is behind them and they can’t be pushed back - on a 3 turn cool down. Tommus’ common helmet currently at level 9 gives me a 375% melee attack with a 4 turn cool down - so it’s a great fight starter and finisher.
Also I don’t think you get penalised for selling off common items earlier on as the way the levels increase in number of common items required to level on means there needs to be lots moving forward to make the next level. Some of mine currently require 400 to level the item up.
Whenever you unlock a new character you seem to get access to a high quantity of common items initially to help level them plus access to specific secret rooms for that character that award further decent number of items for that character. Then there’s common items with high multiples in the challenge levels and regularly you can get common item packs for a lowish level of gems in the store.
Finally request common items if you are in an active guild and you’ll soon fill up.
Hope that helps.