I'm looking for a classic quill stem and found a nice Cinelli XA on the Bay from an international seller. The seller didn't list the clamp size in the ad, so I messaged him. His response was that the clamp was 26.4 but closes to 26.0 when the bolt is tightened. Does this sound right? I need a stem with a 26.0 clamp to fit my bars. What is frustrating is that a lot of eBay sellers don't list the clamp size.

I probably should just buy a Nitto, but some of the classic black stems would look sharp on my frame, which is black.

While some frown upon it, you can always shim a 26.4 to clamp a 26.0.

I probably shouldn't risk it for an international sale, but the stem is NOS, black and the right reach. I didn't realize there were so many different clamp sizes for quill stems until I started searching for one. So far, I've found them listed with 25.2, 25.4, 25.8, 26.0 and 26.4 clamps.

I don't frown, I just do it.
My Holdsworth has a 26.4 Cinelli stem mated with a 26.0 later Cinelli bar. I added brass shim stock until the two clamped up tight. It hasn't shifted since, but it did take a little trial-and-error until I got just the right amount of shim (the stuff I use is .01 mm thick)...you can do the same thing with aluminum can stock.
As we all learned from "Zen and the Art of Motorcycle Maintenance" this is perfectly good, even for a BMW.