"I wonder how they handle the problem where
A wants what B has
B wants what C has
C wants what D has and if you are very lucky,
D has what A wants."
Good question. My guess is that they don't handle that problem, even though its solution is so "obvious". However I seem to remember that there have been "round robin" exchanges here in Dave's Garden. I'm going to look into the Flower and Herb branch of SSE. I seem to remember years ago asking someone who had the Flower and Herb catalog to look for "Crown of Gold Zinnia" and they didn't find a listing for it. But hope springs eternal. If I grow enough zinnias, I might find a gold-based zinnia in my own zinnia patch.
Here is a pink "marigold flowered" zinnia that bloomed recently. Too bad there aren't pink marigolds.