It depends entirely on the figure's history as to whether the rivet has rusted. You can get unrusted ones, but bearing in mind how old these figures are these days it's easy to see how they could have been exposed to the right conditions for corrosion to take place at some point in their existence. Most kids didn't keep their figures in mint condition (although some did; me for one!
), and most certainly wouldn't have worried about getting them wet or exposing them to damp / humid conditions.
Most fixed-limbs C-3POs you'll encounter these days will have rusty rivets, but if you really want a rust-free one, keep looking and you might get lucky for a cheap price, but more likely you should be prepared to pay for a mint condition one (or a baggied one, of course).