I'd also like an answer to this. Adobe's official policy seems to suggest that if you bought, say, CS2 at full price, and then upgraded so you're now on CS6, you can't sell your old CS2 licence because your CS6 licence depends on it.
However, we're CC subscribers, and we're now paying the same price as we would be if we'd never bought any older versions. So I don't see why we shouldn't be able to transfer our licences for CS2, 3, 4, 5 and 5.5 if we want. Anyone from Adobe clarify the position on this?