DO NOT do business with anyone who offers to send you payment for a higher amount than your selling price with the understanding that you'll send back the balance.
Be suspicious of anyone claiming to be acting on behalf of a "client" or another "customer".
Be suspicious of anyone claiming to be from Nigeria or West Africa
If you agree to accept a check payment, then do not send anything, gear or "change" before the check cleared. And be careful, if you manage to clear a fraudulent check then you may be liable for the damages if the bank finds out later. Make sure your bank authenticates the check.
If it sounds to good to be true, then it probably is.