Some help on Weapon Skins!

I am really hoping somebody here (or multiple people) can help me with UNDERSTANDING weapon skins.

Being a long time player (have 2 char. with 4 year old presents) I used to be somewhat "in" with this stuff. However, after an 8 month break from the game, I'm confused on HOW to determine what weapon skins are worth keeping.

I remember the perfect damages and such, and I know for the most part req. 9 is great on weapons. However, how do you know WHICH SKINS are the valuable ones. Is it just something you remember in your head or what? I just want to make sure I'm not selling stuff to a merchant that could be VERY valuable. Please help!

First step you should check wiki: if it's said that the skin is rare, than you should ask for a price check...
another way is just look around in high end sale - you will see, witch skins are "very" valuable

depends on mods also....just take screenshot of the items and post them here

do a little homework no one is going to list all the skins and mods for you here....