You can also use paypal, don't use the gift button. Also if sending a MO make sure you use a USPS order. fill it out completely, and keep your stub. Also, send the MO signature required, so you have proof the buyer got it.
If selling the knife, make sure you insure it for the full value.
Insuring and using signature required, or paypal all add to the cost of the transaction but also help protect you.
A good rule of thumb is if you cannot afford to outright replace the item, or eat a cash loss, you need to insure it.
Some high volume sellers on here self insure, because they have so few problems, the figure that over the long run, it is cheaper to replace the occasional knife than pay the increased cost on every transaction to do insurance.
That is not me.
I first only buy from people with good feed back. I will sell to people without any feed back, but I won't ship until I have the money in my hand.