Did you happen to ding their stars? Although DSR scoring is said to be anonymous, ebay has tools whereby sellers can run various reports, among them one that allows sellers to know how they scored star-wise.
Many sellers don't leave feedback for star-dingers. Without the ability to leave any feedback other than positive for a buyer, the seller's ability to avoid feedback altogether for those buyers is their only way to "un-recommend" those buyers.
In other words, for sellers who run their various sales reports, their "green +" for buyers is their way of "recommending" that buyer. However, buyers who ding stars yet leave "glowing" feedback don't get that recommended buyer feedback.
And since sellers aren't allowed to discuss DSR scores, they can't respond to multiple requests for feedback from buyers who wonder where they're feedback is.
That said, feedback as a buyer is essentially useless since sellers can't leave honest feedback for less-than-satisfactory transactions.