Taken quite a long break from domaining and just been getting back into it recently, so not sure what my opinion is worth, but here you go; I'd hold on to them, or look to register new ones related to the growing cryptocurrency market. Bitcoin, being by far the most popular, has exploded in popularity over the last few years and appears to only be getting stronger. The communities surrounding these are generally quite tech-savvy as well and can see value in strong and memorable domains, whether for investment or trading themselves or simply the aesthetics. This leaves a lot of room for keyword development, I think. I don't own it, but exchange.cc is a good keyword in itself and, for this particular market, very easily brandable and memorable as a platform for trading cryptocurrencies.
As I said, slowly easing myself back into domaining after a several year break, but would be nice to hear other people's opinions on the above. It won't happen overnight, but I think it could be a fairly successful marketing campaign for the .cc extension.
Anyone else for .cryptocurrency?