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.
No. Unless you have an agreement with the buyer in which it states that you have to.
That said, if you are aware of a trademark issue when making the sale, and a problem arises for the new buyer, they might claim that you acted in bad faith and request compensation for damages and lost profits (but they would have to prove that you knew).
I have successfully helped over 300 entrepreneurs and would be happy to help you (after scheduling a call, please send me a message with all the background information so that I can prepare in advance). Good luck
It is the responsibility of the website owner to ensure that they do not infringe on any trademark laws. In this case, it is the buyer's responsibility to perform the trademark search and to use the domain lawfully.
No. You don't need to do this extra work before you sell them.
Though it is the buyer's primary responsibility to verify and validate for trademark infringements, however this does not waive all your liabilities.
If the domain names are violating any trademarks before sale, then you would be accountable too for that extent
You will be liable for damages and claims for any infringements till the time of the sale
It absolutely IS your responsibility for you do a trademark search before you sell a domain name. Otherwise you could be selling someone else trademark property which you have no right to do. Leading to lawsuits and a damaged reputation. Research before you claim and sell and domain.