It seems EBay policy allows for the buyer to place Feedback on every item regardless of any conditions that may be mitigating, even when all items have been combined into one single transaction. This policy opens the way for abuse and blackmail by some unscrupulous buyers.
I have found as a seller that one bad sale or even a sale in which you have acted in good faith can be used to trash your reputation, for example.
You have had 6 sales all to the one person at the one time, If that person wishes they can add bad feedback on every item regardless of any conditions. What makes this wrong is when the Buyer asks for all items to be combined into one transaction the buyer can still leave bad feedback on each item.
The easiest way to reduce the risk of unjust Feedback.
Comment on every single Negative Feedback you receive. Allow for a section in your about me page to refute or explain each and every single negative feedback.
Making a comment in your about me page, allows you the opportunity defend yourself.
In most cases the Negative Feedback is a result of a customer NOT reading the trading conditions, having their Negative Feedback and your full reply posted, can allow new customers to reach their own conclusions. This will show you have nothing to hide and are prepared to stand up for your rights as a seller.
The best advice I can offer is to List all trading terms and conditions clearly and in detail.
The customer is not always right however as a Trader you should go the extra mile to make sure your customers are happy, if you have made an error say so. Offer your customer a gift or a little extra this is the best way to avoid the big red letters on your profile.
List your trading Conditions clearly
Reply to your Customer,
Inform all Buyers that any comments will be posted.
Answer every Negative Feedback clearly.
Offer a free extra if you can.
Contact your Buyer after you have tried to rectify the issue. They may change your feedback
Contact EBay under a very few conditions you may have the right of removal of Negative Feedback