WRT item sets, I don't sell any outright just because i don't know if they'll change them in the future to be better, but -
Situationally usable:
Destroy - really only need at most one champ with this set for scarab, if you have no other max-hp damagers.
Fury - personally haven't found a use for it, although I'm sure there are some specific champs that benefit
Curing - same as above, I guess technically on a dedicated healer like Scyl this might be useful, but I haven't really found any use for it
Reflex - very situational, for champs that absolutely want to be using a specific ability as often as possible
Immunity / Untouchable - could be useful on specific arena teams, but I really haven't found any that would prefer this over basically anything else
Regeneration - maybe some sort of weird cheese arena team with super high defense/resist and these items to stay alive, but since regeneration only triggers on your turn, i just don't see this working
Can't think of any use:
Cursed - technically I suppose it could be useful in a very specific arena team, but honestly, I find you're better off just killing them outright instead of trying something weird like this
Toxic - if it were 5% instead of 2.5%, I could see this having some use for certain CB teams, but at 2.5% i just don't see a use
Daze - this does have a higher chance to land than stun, but given it gets removed on damage I just don't see any reason to use this over stun