I agree with CraigR. The only thing I would add is that, in the sentences "Could THAT be it?" or "Is THAT it?", THAT is like 그것 or 저것, so if you (the speaker) picked up an item and wanted to ask that question you would say "Could THIS be it?" or "Is THIS it?" because THIS is more like 이것. I'm not 100% sure this is right, but I think "Could that be it?" is like "그것은/저것은 워하는 물건이 있어요?" Sorry if this doesn't make any sense! If you have any questions please ask and I will try to explain better =)

Imagine that you're searching for something with a friend and you find something that may be what you're looking for. You could say to them 'Could that be it?' It could be interchanged with ' Is that it?'