Question about equipping weapons/armor on FF III.

[Britton]

How in the hell do you tell if what a weapons/armor seller is offering is more powerful or weaker than what you already have equipped? Also, in the instruction manual, it says when you're in a shop buying such things, the characters on the top screen will wave if they can use the weapon/armor you're looking at, but it seems I can equip everyone with anything. Right now (early in the game) I have all 4 of my part ymembers using long swords in each hand, even though back in the shop it says two of them can't use them...