In some cases, literal exchanges are possible. AA->HH is a fairly common one that actually doesn't punish you too bad on raw value, assuming you have a strong near-term use for the HH points lined up.

I won't argue for or against bartering or selling points/miles here, but if you were to do that you'd have to assess the rough dollar value of your award vs. the value of whatever you are getting back. Every one of these 'currencies" is different, and I can't think of any hotel award that are 1:1 exchange for their point to an air mile.