1. Have a contract
2. Make sure their money clears the bank first. Feel free to provide the purchaser the license number to verify the transferability.
3. Use escrow if you must
4. Don't sell to anyone who isn't close enough to sue or report for wire fraud
5. Easiest - sell to someone you know or can verify is a real person or bona fide operation who wouldn't want a trail of written misery and woe to follow them.