I think it's a bunch of hooey to inflate the price...

I think official seed potatoes are chosen to be disease free and may be treated with something to protect them. That said, my folks grow and eat "volunteer potatoes" that show up in their garden and they are just fine. So maybe your yield might be a little less. CSA potatoes sound fine. Your local extension would know the difference, if there is any.

The only exception I know of is that you can't stick rose cuttings in a grocery store potato to root new roses.

The growth retardant with which grocery store potatoes are treated to prevent sprouting will kill cuttings.

(When I was little, we liked to grow pineapple plants from the cut-off tops of grocery store pineapples!)

Ginger and turmeric roots will also grow. I've not tried turmeric yet, but have best success with an organically grown ginger root that has just a few greenish looking nubs that show promise of becoming buds and new growth. These are tropical, so something I have to plant in pots to bring indoors in winter, but in your Zone 9 climate, they should be happy year 'round.

Any part of a horseradish root will grow ... vigorously.

The bottom of a bunch of celery will regrow, but I've never tried growing it for long, so don't know whether it would eventually produce something harvestable/usable.

I've grown garlic from supermarket garlic, but all things considered, don't mind paying the higher price for heirloom garlics, especially the hardneck varieties.

1. Grocery store garlic is usually a softneck variety, as it has a longer shelf life, and may have been grown anywhere in the U.S. or even in China;

2. Hardneck garlics are said to have more of the healthful substances for which garlic is known;

3. I buy from local, organic growers, so I know the planting stock is well-suited to growing conditions in my garden;

4. There are so many wonderful varieties of garlic out there, with different top notes and finishes! Mild, strong, hot, spicy ... different kinds suited for all kinds of cooking: French, Italian, German, Spanish, South American, Asian, Cajun, etc. (Not that I grow them all.)

5. The return on investment is pretty good. Since each clove will grow a whole new bulb, you can get anywhere from 6-12 new bulbs from each single bulb you buy.

Another nice thing about the garlic is that it's best planted (here in Zone 7) in the fall, when gardens are usually "put to bed" for the season. It's 28 degrees out there and my garlic, which has already sent up shoots, is fine, and will be throughout the winter, which can get down to the teens and even single digits occasionally. It will have a few growth spurts during milder spells, then growth will really take off in early spring, scapes will be harvested in mid-spring, then the bulbs in late May or early to mid-June, depending on our weather.

Yes supermarket selections aren't necessarily the same cultivars you want for your own use in your area - they may not even be locally adapted, if shipped in from some distance away.