Try to take the full payment at the time of the delivery. Remember to use a safe location to meet.
Watch out for fake currency and money orders. Be aware that banks will not honour fake currency/money order and will hold you responsible for it.
Verify the buyer's credentials if there is a request for a bulk order. Ask the buyer to provide photo ID and proof of address.
Ensure that your product meets the features you list in your ad to avoid any misunderstanding with the buyer.