The seller is 10MFan who is a big dealer in mouthpieces, well known on SOTW. His real name is Mark Sepinuck and he has played quite a large part in developing the overheated market in used mouthpieces.
He seems to get a lot of vintage mouthpieces through his hands, from the very unusual to the quite common, but desirable. His prices do tend to be high, but he is good to deal with, friendly and helpful.
He has also recently developed his own new line of tenor mouthpieces, first in ebonite and more recently in metal. These have been heavily hyped and praised over on SOTW, but you do see quite a lot of them reappearing on the used marketplace over there.
I would guess that whoever had Gerry Mulligan's musical equipment has decided to turn them into cash and Mark Sepinuck would certainly be the best person to maximise the price they get for mouthpieces. He knows how to use eBay and I wouldn't be surprised if he gets a high price for some or all of these pieces, although it may take a while. Even just pricing them high can help make people think they must be very valuable.
I saw Ronnie Ross' baritone mouthpiece for sale a couple of years ago for basically the same price as any other old Berg Larsen, but unfortunately it had already sold before I got it. I also thought I had bought Wesley Magoogan's alto mouthpiece from the man himself, but that deal went wrong.
Rhys