There are two important things to consider: A cashiers check is more difficult to obtain than cash, and it is possible, contrary to popular belief, to stop payment on a cashiers check.
That said, I still wouldn't jump to the conclusion that it's a scam. Just tell the person you can only accept cash then and there for the item.
If they can not meet in person (live too far away, or whatever), it's still safe if you do the transaction with paypal, so long as you haven't misrepresented the condition of the item -- you will receive the money before sending the item, and you can issue a tracking number to paypal so they're protected by paypal's buyer protection policy. If you're going to do that, however, you may as well just list the item on ebay.