Thank you Poeme
Yes, I do sell them and you are welcome to visit my page for more info on them at
http://fluteragiouscreations.blogspot.com/
Honestly I would say no that they do not make any difference in the sound. They are purely cosmetic.
You may be interested in also trying out the locking crown system by Nagahara. I tried one on my headjoint and was surprised that I did hear a small difference. I don't think it was enough difference to pay the asking price for one though.
I have not gotten to try the Bigio stoppers and was disappointed that no one had any at the flute fair to try out. A friend of mine said she loved the sound and it made her headjoint sound like crystal but ended up selling it because it was just too heavy and weighted her flute down too much.
If you are in the US, I do know that Phil from the flute center of NY carries the Bigio stoppers.