If you are only around level 100 in skinning, you just need to carry on killing beasts, gaining the XP and improving your skill at the same time.

A good investment is larger bags, as well as an AH Mule (a character that sits between the mailbox and the AH) and the Auctioneer Add-On.

With these things, some time and a few quests to boot, your skinning will get better meaning you will make more money as you get better leathers.

I found on my server Alonsus i didnt make much until i was selling stacks of Medium Leather or higher, however your server may vary.

Keep at it and you will reap the benefits after a while, skinning can make you a ton of gold :)