I managed to get Gerald to stock two +1 Melee Damage swords. They were a massive boon to starting the game. Besides that, it isn't worth it.
Merchants have pre-determined items, and also stock certain types of items besides that. The pre-set items are always the same and the merchant will always have them until you buy them, whereas randomized ones are selected every time you level, and each level-up replaces all the previous levels' random items.
