On one eBay purchase, I received a non-working computer component. The seller was not cooperative until I threatened to take it to dispute resolution and leave negative feedback.

On another, the seller was verbally abusive [in writing] and, when I complained to eBay, not only did they refund my money, they banned him from selling with them.

If you are going to leave feedback of any sort, do it within 30 days of purchase or the opportunity may go away.

The system works if you do it right.

PS, always use PayPal.