I didn't see our Blackberries (8100 Pearls) listed from this particular seller on eBay, so I took a chance and ordered one I THOUGHT would be compatible. It wasn't (I didn't realize the plug size varies between 2.5mm and 3.5mm in the various models). I got a 3.5mm version and the Pearl only has a 2.5mm plug. Radio Shack (and others, including the Verizon store) have adapters, but I also found out that there must be a 3-band plug, not a 2-band plug (small rings between the metal segments in the plug itself), among other things.
I left top-notch feedback for the seller because he kept up his end of the transaction wonderfully (shipping was included in that under-$5 price and it only took 2 days), but wrote him a note to suggest putting an additional line in the description of the items noting that various models have different sized plugs. His response? He just said send the two back, including the one I'd opened and tried to use, and he would send me ones specifically made for the 8100 - and he didn't even wait to receive mine back! He just said to "stick a couple of bucks" in the envelope to cover his shipping cost on the replacements (I sent him $5).
I had a 2-hr each way trip last Saturday, and it was the shortest 2-hour trip I've taken in a long time - listening to a seminar and lots of music from MP3 files was great (sound quality is surprisingly good for this inexpensive device). It doesn't have much in the way of bells and whistles - the button only answers a call, thereby shutting down the MP3, and when the call is over the MP3 starts right back up where it stopped before the call. I called my wife at home and she compared the sound from the clip-on microphone and my blue tooth and said the blue tooth was better, but not by much - the inexpensive device was quite acceptable she said.
So the bottom line is, write that particular seller first if you don't see one for your exact model - he may have one. His service can't be beat, and with shipping included I have no idea how he does it. Thanks to Cynaburst for the link!
Chris