Usually when someone is planning on blackmailing you their emails are extremely rude and threatening. For example you mail the package the same day as winning bid, you send with delivery confirmation and return receipt. You receive the confirmation and customer's signature back and think that all is well. A day later you receive angry email from customer that they did not receive item and if refund is not sent immediately to their paypal account they will leave a negative feedback. The customer is not asking if the item has been shipped, or stating they are unhappy with the item, only that they want a full refund immediately or will leave you a negative feedback. In cases like this I always let them know that I have their signature and delivery confirmation from the USPS stating their package was received, but I would be happy to file a claim with the USPS for them and the USPS aka Government will investigate the possibility of fraud. Usually that works as no one likes to mess with the government
Every now and then I receive an email stating "I hate to leave you a negative feedback" BUT .... I always reply if there is a problem with the item please let me know WHAT it is and I will replace or whatever is necessary. There is no need to threaten me with negative feedback. I mean if I don't respond to your issue I can see threatening, but to start off the first contact with "I'm gonna neg you" is a little unfair. Just as unfair as giving a negative without even contacting the other party first.
I can understand the buyer's frustration as I buy on ebay too. I have been ripped off as well, so I know how it feels, and that is partly why I strive to provide excellent customer service. These days it really does pay to shop with ebay sellers that have stores on ebay, as you know they will still be there tomorrow.
What to do once you have been blackmailed. Ebay's feedback extortion info page is found here
eBay will even remove the negative feedback under certain conditions. One way to have it removed is to check the other parties contact information. If their contact info in not correct, ebay will remove the negative. A mutual withdrawal can also be requested, where as both parties agree to remove their negative feedback. Info on both can be found here
Another way to have negative feedback removed is if the member is suspended under one account, re-registers under another. That feedback can be removed. Here is a link on other valid reasons to remove feedback.
If you think you qualify for feedback removal, use this form to request the removal from your profile.
There is a web site that is very useful as it list all feedback for a particular member, even if it has been removed. It is a good idea to check the person out to see if they have a habit of leaving bad feedback (many do)
You can check all the feedback they have left for others, and feedback that has been received. It even list any that have been mutually withdrawn. A very useful tool that warrants a place in your favorites.
I hope you have found this info useful, and remember it is not the end of the world when you receive your first negative feedback.