You may be able to get decent stuff for them, but nothing very rare. This is because despite being rare to encounter normally, they spread to your other Pokemon, so there's a high supply of them in the online "marketplace".So a Froakie with it is a good thing...?
This would be my third one then, my other ones are Zweilous and a Trevenant. Are they of value in any way in terms of trading? I don't competitive battle much...
A Pokemon will eventually be cured of Pokerus (after 2-5 days I think). To keep Pokerus on a Pokemon, keep it in the PC. The Pokerus time ticker counts down at midnight, so you can have a Pokemon with you all day, then deposit it in the PC at midnight or before you go to sleep.