Negative feedback is the prime reason why crooked sellers and buyers are soon excluded from the eBay marketplace. Negative comments are red flags warning future buyers that a seller has demonstrated a total lack of responsible behavior in prior transactions and can't be trusted. You should always try to resolve the dispute before entering negative feedback, most sellers will go out of their way to make the situation right. You should always leave negative feedback when the situation calls for it. Doing so will protect other eBay sellers from getting ripped off. Always leave a negative if the following occurs:
- Seller refuses to sell
- Seller fails to send item after receiving payment
- Seller misrepresented the product and refuses to refund.

You have a responsibility to leave negative feedback to warn other eBay members of potential problems dealing with the seller. Negative feedback counts as a –1 and is deducted from your feedback profile

Tip
If you are concerned about getting hit with retaliatory feedback from a user wait until day 89 to post the negative feedback rating. Since you can't leave feedback after 90 days this won't leave them much time to retaliate.