Some sellers are greedy, some are unscrupulous, some may even be outright frauds, but very few, if any, sellers may be "Evil"
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On a more serious note:
There are a number of ways to solve a dispute with a seller than leave a negative feedback.
First write a mail to the seller, after 2-3 days send a reminder. The seller may have not received the first mail or may have deleted it by mistake.
If after 7 days of letting the seller know of your complaint the matter is still not resolved, than lodge a dispute (INR - item not received or SNAD - significantly not as described).
The seller more often than not will resolve your complaint within 7 days of a dispute being opened. If the dispute has been resolved, please be gracious enough to close the dispute. Remember a dispute can only be closed by the Buyer (not the seller and not by eBay).
If the seller does not resolve your complaint within 10 days, you may choose to escalate the complaint to a claim for re-imbursement under eBay Standard Purchase Protection Program.
http://pages.ebay.in/help/tp/isgw-buyer-protection-steps.html
eBay protects all purchases upto Rs.10,000/-.
Leaving a Negative may get you a Negative in return as you say but if you are smart, you can amicably resolve the dispute with the seller and if the seller chooses to not to resolve the complaint, you can claim re-imbursement from eBay. When you do so, eBay may take certain actions against the seller, viz. warning him, putting restrictions against his account or even suspending his membership.
So the smart way is to follow the dispute resolution process and not file "Negative Ratings" off the cuff without giving the seller an opportunity to resolve the issue.
There are some Feedback (Ratings) changes in pipeline which will neutralise Feeback by any User who is unwilling to participate in the Dispute Resolution Process.:
http://www2.ebay.com/aw/in/200506.shtml#2005-06-24192504