It is always best to meet the seller face to face and inspect the goods before you part with your cash.
If the transaction must be done by post. Then don ‘t send cash in the post, never use MoneyGram or Western Union they do not leave a paper trail, if the seller insists you pay by these methods don’t it could be a scam
If the seller takes PayPal, then that’s an easy method, if you send a personal cheque then the seller will be entitled to hold off posting the goods until the cheque is cleared. A bank transfer is another option.