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.