You can telephone the bank that issued the cashiers check and they will verify that it is authentic based on the payee, check number, amount, etc.

If you use a national bank with branches near the seller (e.g. Chase, Wells Fargo, etc.) you can go to a branch near the seller and get cash or a check while the seller is with you. That tends to dispel any concerns regarding authenticity. It also allows you to complete the transaction in the security of the bank branch - less opportunity for any problems.