What items to keep and what to vendor?

depends a lot on your playstyle and skill level... so no list is going to work for you.

I save far less than I did two or three years ago. I realize that I can make do with "random" just about as well as I can with "cool". I pretty much save only the "really cool" items for potentially using for future characters.

It doesn't seem like there's a hard and fast rule. I've been selling everything I don't need if it's worth more than one or two alchs, but I'm the kind of person who doesn't intend on making a dozen characters and equipping/experimenting with them all..

Do you just play the odd character or two and concentrate on end game or do you like trying out lots of builds.If the former you probably don't need a lot of low level uniques but if the latter levelling uniques are useful.

It really depends on your 'needs'. If you're SURE you'll never want to level a 'different' build that depends on dedicated weapons, you can cut all this stuff out from your list/stash. Endgame weapons with >>100% damage, high base 6 socket and of course 6-links may be worth to keep, anyway...
For casters, some variety of +skill weapons may also be useful, and there are also helmets with + minion gems to have a look at.
Armour equip parts depend on your defensive concept (do you want armour/evasion/ES?), and especially for the body: what socket colours do you want/need...
Capable equip from this demands then is sorted out by some generic stats: life/es affixes, flat phys damage/attack speed, % (spell) damage/cast speed, resists, attributes (and not to forget move speed on boots, and also to consider: free affix slots for crafting options).
Weighting this can shrink your stash demands significant, and the less you (may) need for twinking, experimenting and other rerolling the less you 'have' to keep.

(I also 'struggle' a bit to decide what my chars 'really' need of levelling gear, because I have to do it very rarely. From regular char building in challenges it 'seems' that there's a point where it slightly 'start to matter' - but more because of sockets and colours then for stats) ;)

What stats to look for:

Defensive items except body armour (Gloves, Boots, Helmet): 80%+ total resist, decent life roll (T3 or better maybe?). Boots need movement speed on top, gloves maybe some offensive stat, helmet maybe accuracy.

Body Armour: Has a hard time competing with unique chests which are often best in slot or at least close to it and easy to aquire. Very high life rolls (100+) and 80%+ resists are an absolute minimum to sell. High armour or evasion is a nice bonus.

Classic attack weapons: High physical % damage or flat physical rolls, attack speed, maybe crit chance (bows, daggers). Usually you need a combination of those to achieve at the very least more than 200dps on a 1 handed and I don't even know...350+ on a 2 handed weapon? My numbers are probably too low because there are cheap unique items that rival them.

Caster weapons: High spell damage/ele damage rolls, cast speed, maybe a combination of crit chance and crit multi, flat ele damage to spells as a nice bonus.


Jewelry: Here is where it gets complicated. High life roll and at least 60%+ resists is a good base line again but for the plethora of offensive mods these items can roll you need decent knowledge on what builds exist and are in the "meta". Best of course if the mods make sense together: cast speed + spell damage, crit chance + crit multi. Then you can deduct yourself that the item might be useful.

If you want to find out prices of an item, use poe.trade and work with ranges. You can for example search for pseudo mods of a total resist % on an item. If I have an item with 40% fire res and 40% cold res I will usually use that search funtion and search for items with a total of at least 80% resist. Maybe set a range from 70 to 90. You get my point.

When it comes to currency, uniques, maps etc. download poe trade macro. The prices on these you can check by simply pressing ctrl D.

Also google for poe affix sites. On these you can check out what "high tier rolls" are.

This is a difficult question for all beginners. There is probably no simple answer, one of the bad things about poe is that it's a steep learning curve.

Except the good writeup @Burningham did I can advise you to check what items are worth on wow.poe.trade. For unique items you find it's pretty easy to getan estimated price, for rare items it's harder - you can try searching for an item with similar stats. If items with similar stats are worth little or very little, you may as well vendor it, because you will always be able to buy a similar item if you need it, while the hassle of trading can be too much.