While doing my usual survey of theremins for sale on eBay this morning (10/1/2020) I came across a rare sight: there are currently two Etherwave Pros and three vintage Moog Maestros for sale all at the same time from different sellers. The Pros are as usual priced out of this world.
I don't want to post the links but if you search by the specific name you should see them, although for some reason using just searching for "theremin" does not always find them.
I usually do this type of search several times a week, and this seems like a pretty rare event. I'll be curious to see if these listings stay up or end with sales.