In Path of Exile, as a new player, one of the hardest things for you maybe to understand which items are good.
Most of the build-enabling unique items are worth some good currency, but some players still find it very hard to understand which rare items are good and worth keeping/selling. Are there any community created tier lists or cheat sheets for uniques and rolls on rare items?
In fact, it definitely depends on the build and who is planning to use it, but general rule of thumb is high flat life and elemental resistances on all armor pieces.
Things specific to certain builds like flat physical damage on gloves are noteworthy too, or to minion gems on a helmet. Easiest way to tell for newer players if an item is worth anything, just check the tiers but holding alt (and having advanced tool tips enabled).
And search up similar items on a trade site, never look for exactly the same, but take the 2-3 highest teir mods and put them in, and set the minimum value to a few lower than what dropped.
And since you mentioned uniques, a good resource to tell the value of those is poe.ninja, it has a tracker of each item of the past 7 days that is not always accurate, but gives a decent idea of where the price is around.
Rare items function the same as magic items, except they have at most six affixes – three prefixes, and three suffixes. Found rare items or items made rare with an Orb of Chance or an Orb of Alchemy will always have at least 4 affixes, but it is possible to get only 2 or 3 by using a Regal Orb.
A rare item with less than six affixes can gain an additional affix with an Exalted Orb or through Master crafting, chaos orbs completely reroll a rare item’s affixes. For more poe guides, you can visit U4GM.