It used to work that way, back in the day. In fact, when I first started with ebay, you could leave feedback for a transaction that was conducted on another site! One of my first feedbacks on my main ID was for a trade I made with someone on a Beanie Baby collectors' site.
Then they switched it to ebay only feedback (as it should be), and every feedback counted. Now you get one feedback number per buyer. I don't agree with it, because many times I have had good experiences with a seller, and five transactions later have a really bad one, but whatever feedback I leave at that point does not affect that seller's rating, as they have already gotten my "one." I believe that every transaction is different, and should be treated as such.
And yep, I have the repeat customer problem too, on both of my accounts. I have seen users with a feedback rating of 25, and when you check their feedback page, they've done 2,000 transactions. What really s u cks about this, is if your feedback number stays lows, a single negative will kill your percentage. Very unfair.
Kathy