Just a couple of things I didn't see in the older thread. Starting in the 90s you start to see way more parallel sets and insert variations. Sometimes it's difficult to tell the difference between the standard Chrome and the Refactors especially the earlier versions for instance (newer versions spell it out or have a true product id number) This could be the difference between a $0.15 card and a $20 card. Other parallel sets can be tricky too.
Today for instance I was going through a ton of 80s and 90s mostly junk.. but I hit a Pacific Paramount Edgar Martinez. I happen to like the 90s Pacific cards so I set it aside... turns out it was a Paramount Blue insert... a variation that, according to Becket, greatly increases the value. A base card in the set is listed (Beckett High) as $0.30 and the Blue variation is $12.00. The only difference being the shiny blue foil. Or take the 2003 Bowman or Upper Deck Collector's Choice... the only real difference between a card worth pennies and a $10 card is the color foil script signature.
Then don't even get me started on the some of the glossy sets that are virtually identical to the standard set.
The other thing to look out for starting in the 90s are cards with a limited run with the serial number printed on it... like 45/100.. obviously the lower the better... you get some 90s cards with serial numbers in the 10s of thousands (especially the Classic series). There are also a lot of game used relic variations. Those are a little more obvious. And signatures... those are pretty obvious as well. They usually spell it out for you n the back of the card with a statement about it being an authentic auto or a game used memorabilia card.
The variations and inserts as you get into the 2000s can be maddening. lol.
I was out of the collecting game for about 15 years and these are things that were somewhat new to me and might help you out.
Good luck!
-Ben