Okay, so I just started welling for the first time, and I have a bunch of tier III and IV items that I've been saving (mostly because I was hoping for some decent secondary drops) but I don't really know what mods are worth keeping on them. Obviously "3 extra item slots" fetches a good price, but what else is in demand? Also, at what level? Thanks guys.

Also, in order to keep my mage at level 64, should I kill him before he levels to 65?

if your welling all the low level tiers at 64 mostly you will be looking for double drops like you said, and in order to keep him at 64 you must kill him before he hits 65 But some good stats on like IV drops would be 15+ strength/dex 65+ EE

Mostly anything that level will only be worth something if its perfect or have many mods on the same all decent numbers

This thread should help you out:

http://forums.d2jsp.org/index.php?showtopic=28582588&view=findpost&p=174311528#entry174311528

Thanks. Are those stats good across the borad? IE can charms armor and weapons all get the same level of strength?

yes its all the same, You will not want strength on weapons though