mojos gos upto 405 max damage, appearing on high level "snake" type. something decent would be around 100-350 damage. (For inferno, low level items obviously have a lower cap) This is by far the most important stat and if it doesn't approach this range nobody will want it.
Other stats too look for are critical hit chance, specific skills, mana regeneration, bleed damage and obviously vit and intelligence. It is kind of hard to find those things because the auction house doesn't sort the damage and people auction things nobody wants regardless.
A shield wouldn't be that different from any other shield, so +block, highest block amount, allres instead of the damage mod in addition to those thing mentioned above.
You want an off-hand with 1) 250 max damage and beyond (ideally 300+ to start) 2) crit chance (6%+) 3) intellect 4) vitality 5) mana regen only if you need it for your specific build 6) + skill damage if you can get it but not an absolute necessity. Having a socket is nice too for added intellect.
As Chaosmage said, if you want a shield you just want one that has the usual high defensive stats + high block.
my problem is when i farm, i find many WD off-hands and so far, i have vendored them ALL. This is because I generally don't know what they're worth and don't have the time to research all of them...
shields, I know the value off for the most part...
i have a bad feeling out of the 50-100 WD off hands I have found, i must have vendored something i shouldn't have. this is extremely valuable information to stop me from making silly mistakes.
for the record, my WD is level 38 but, hes on hold right now and a secondary purpose of this thread for me is if i find something "good" but not quite sellable I can mule it to him for when he hits 60.