I would recommend you buy anything that makes your build better because it will help you a lot in the campaign, and many relatively good items are dirt cheap. But don't buy anything too expensive (500+ gold) unless you are absolutely certain that the item is worth getting. Multiplayer chat is a good place to ask for second or third opinions.
There's no "need" to buy any items unless you find something you really want during the single player campaign. It all depends on how you want to experience the game. If you're playing any mp, you might want to look into buying items since cohesive builds there are more important.
As the others said. For expensive items there is a Rate my Randimar's Rarities thread to get advice on specific items. Some common and uncommon items, like Blister Stones, are cheap, very useful, and available in the shops several levels before they start dropping as loot.
I would up your damage for your warrior(s) at the Goblin Bazaar, I hit a steep power curve at one point and found the SP much easier once I loaded up there.
If you have a few hundred gold to spare, and you see something nice, buy it by all means - but you don't need to. Retain some gold for revives and buying new characters - apart from that, you don't need gold so feel free to spend it!
