Consultant to registrars, domain portfolio managers, hosting companies, registries, resellers and the domain aftermarket.
Having worked in the domain name industry since 2001, I would always suggest a trademark search before acquiring names. Please keep in mind that when registering a name, you agree to the Uniform Domain-Name Dispute-Resolution Policy (UDRP) as part of your registration contract. This means that you agree not to infringe on rights of others with the registration of your domain.
When selling a domain name, it really comes down to your contract/agreement with the buyer. Depending on the details you might be held liable for any potential trademark infringements at a later time.
DIsclaimer: I am not a lawyer, it would be best to consult with a lawyer.
From a SEO perspective the domain shouldn't really make any difference - or at least not a major one. As Google often emphasizes they try to rank by relevance for the user. There's one main exception, which would be ccTLDs (country level domains). And while .CO is a ccTLD, it's considered a generic domain by Google.
There were some TLDs that would negatively affect ranking, but .CO wasn't one of them.
I do think you should always try to own the .COM though - as domains get passed on by many means, i.e. word of mouth etc. Users are used to seeing/hearing .COM more so than .CO, so they might just try to visit the .COM instead of going to your site.
I'm sure they are losing some traffic clarity.com - which redirects to clarityoss.com.
Someone from the Clarity Team would be best suited to answer this, but I think the association with "FM" -> Radio -> Talkshow is probably why they picked this TLD. Don't think they are based in Micronesia ;-)
I've also got a FM domain since it matches my initials, how could I resist.