I always made that tough choice like this: compare your items of the same type (i.e. all weapons, all helmets, all armor, etc). If it's not one of the best 3 items, sell it. In addition, if it's quite cheap even if you thought it was rather good, there must be a reason why your other items are rated much higher --> sell it.
I must say that just selling the junk of the junk usually gave me enough money to get by. In addition it helps to "plunder" the army stock when you're allowed to take whatever you need from there. Then sell it back to them
