Wow! A dozen varieties is a bunch. Other than my own garden I've never seen that many in one place.
Let me make some guesses:
1. Several zucchini. There are different versions of the long green we're all familiar with, varying primarily by the number, color, and size of the stripes. There are semi-fluted ones. And there are round ones. The last few years have seen a plethora of colored zukes as well, including yellow, white, gold, and what is called black but isn't.
2. Pattypan (i.e., scallop). There are about a half-dozen colors of these; solids and stripes.
3. "Yellow" summer. These come in straight-neck and crooked-neck versions, with the crooked-neck the historically earlier---it's preColumbian---and better tasting. Then there are some modern hybrids, like Zepher, which are bi-colored green and yellow.
4. Offbeats. Varieties like Lemon Squash, which resembles its namesake in size and coloration.
5. Marrows. Not all that common in the U.S., marrow squashes should do well in the SF area, because they do better with cooler, damper nights. If this vendor is specializing in summer squashes, it would't surprise me at all to find he's growing marrows.
I don't know any one place you can find pix and descriptions. I would look at the websights for Johnny's Select Seed; Baker Creek Heirloom Seed, Victory Seed, and the SSE public catalog.