Curing, Avenging, Retaliation, Life, Defense, Attack.  Of course if you happen to drop an absolute banger one roll it for potential.  There's so many good sets with variable options I just dont find mant offset pieces anymore.

Personally I wouldn't sell any gear set item. Roll them if they are worth rolling, and sell if they don't roll well.

I suppose I would keep a 6* Mythical bottom row piece with perfect stats and a double speed hit if it came along, but ordinarily I auto-sell these sets, and have the geat filter set for it: Life, Offense, Defense, Crit Rate, Resistance, Retaliation, Lifesteal, Destroy, Toxic, Frost, Daze, Avenging, Stalwart, Fury, Curing, Immunity & Frenzy.   

Here is an essay for your reading pleasure:   

As the menu of 9 piece sets continues to expand, I would much rather keep more pieces of fewer sets because that makes putting together the best builds easier.

TBF, I have never been a fan of using lots of broken set pieces, and as the set bonuses have gotten more and more powerful, they are less and less viable all the time.

Here is an example of how my thinking on this works:

Ignore Defense Gear:

(1) Savage, Lethal & 6 pc Merciless all serve a similar role.

(2) Merciless is a dramatic upgrade over Lethal & Savage gear, but getting the 35% Ignore Defense requires at least a 6 piece set. 

(3) Lethal is a straight upgrade over Savage due to the added 10% Crit Rate. 

(4) Because Inventory Space is always at a premim and there is a constant influx of Merciless & Lethal gear, I am slowly but surely eliminating all Savage gear from my inventory and keeping a new piece of Savage gear almost requires it to be 6* Lego (or better) with perfect stats.

I would much rather keep 5* Lego Merciless gear than 6* Epic Savage gear because Mercilees gear comes with extra stat bonuses and I want all my options open when trying to hit 6-9 piece sets.

Keep only the top 4 Swords, helmets, and shield. You will never arm more than 4 heroes at once so find the best four for each set and sell the rest.

No flat stats gloves boots chests.

https://omegle.onl/

No shield/bolster sets without hp% substats

https://xender.vip/

If you still need to sell remember “if it don’t have speed, you don’t need”.

Still need to sell? Then start getting rid of rare 5 star