Why use signed integers?

I've noticed that, in quite a few MMORPGs, the stackable item/money cap is (2^31)-1, or 2,147,483,647.

But why use a signed integer? Do you really need negative amounts of items?

Why not use unsigned integers and then have the cap be (2^32)-1?

So why does EOF need to be -1? Why not make it ASCII 3, 4, 25, or some similar control character?