It's a bit of a mishmosh, but start stalking the trophy room, and look at the stats of the items there. If items you find get many or full jackpot with these stats, put ut out for a price check.
Personally i pick up almost everything ^^ mainly because i'm a hoarder, secondly because i am curious of nature, and thirdly because i am a gambler of nature xD
But then you have to be careful too, because you may find something valuable, but a) Those that are rich may not want to you get rich, so they may tell you it is not worth anything, or b) they will offer to buy it cheap.
The trophy room
https://forums.d2jsp.org/forum.php?f=103
But also, common sense can get you far. If a magic weapon has stats that give it good damage, like almost perfect roll, that is worth checking out.
Affixes
https://classic.battle.net/diablo2exp/items/magic/
D2:R