If the speakers are in good condition I'd say nothing higher than $450 for the RC-30s and nothing higher than $650 for the RC-50s. (The highest being with shipping if you're getting them shipped.)
Obviously it's your call if it's on the border of what I said above, but don't let anyone fancy talk you into buying their speakers for much more than that "because they once costed $2000 MSRP" and jazz like that. I've seen alot of people on Craigslist putting stuff like that to make it more impressive. Chances are people that have these RC-50s and RC-30s got them during the ridiculous firesales at the beginning of this year.
Edit: Those RC-10s do sound MUCH better at ear level, don't they?