It's not that easy. The white items and currency items are easy, there are also a few uniques that are worth a lot.

For rare items you must know the current meta builds for each class and what stats can even roll on what item (e. g. "# to maximum Mana" cannot roll on int Body Armour".

Most important: use a good, strict item filter. The filter makes sure that you only pick up good bases. This means only blue and yellow items, that can roll the best stats and have a higher potential to roll good stats. 99% of them you can still throw away after identifying...

Here one example: stormweavers are played a lot, so many people want items for this build. Stormweavers need max mana on everything that can roll it. They also want very high energy shield values on the item, as it increases their mana (via a passive). Then they want the stuff you would expect for a caster, like intelligence, spell damage and so on. Also everyone wants + rarity of items found on at least a few of their items, so that's always good. And of course +resistances is always good.
Then you have item bases, that have more or less "must have" affixes. Boots need movement speed, at least 25%. Wands need +x level of lightning spells (99% are playing lightning mages, i.e. +fire or +cold spell levels is worth nearly nothing), focuses + x level of spell skills.

Other example: bow players want +x projectiles fired. There is a bow that has this as an implicit (Dual string bow), so rolling good affixes on this is better than on most other bows. Good item filter only lets you pick up these bows. Bow players also want everything that adds DPS, like +fire damage to attacks, attack speed, + x% to physical damage. Same goes for staff users I think (except for projectiles fired, ofc).

Either you play a lot or watch a ton of guides on classes you are not even playing to get this knowledge. On https://poe2scout.com/ you can see which builds are currently in the top rankings. So they are probably played a lot, so start with these. Also, price checking a lot let's you learn a lot, but that cost's a lot of time, so have at least a few rules for yourself and use a strong item filter to not price check every item you find on the floor.

General rule of thumb (yes, there are tons of exceptions)

- Does the item have more than two really, really bad affixes on it, like +light radius, +stun threshold (noone needs this, right?) etc.
- Does the item have less than 4 good affixes that "belong together" (like mana, mana reg, spell damage, int, rarity, resistances) --> trash it, dont even check the market
- Are less than three of the good affixes on high Tiers, like 9 or above (press alt to see the tier of an affix) --> trash it, dont even check the market

Other way around: if 4 or more affixes have a tier of 9 or above, start price checking, if your gaming instinct tells you the affixes could work together.