I'd say 6 weeks so that the roots can get established. You don't want something dying as soon as a customer gets it, plus you can make many more divisions if you pot ahead of time. Selling something just dug up, unless customers are told that, isn't something I'd want to do.
Some people dig them when a customer is there, which is a different thing.
When I sell a gallon perennial, I want a full plant that will be sure of success, and want enough roots established to hold the soil together. If you divide, pot and sell all in the same week, when the customer goes to plant it, only the division will come out of the pot. It may not be good for your plant reputation. I'd say offer a plant for sale that you'd like to buy.
Ann

Six weeks it is. I would think "offering a plant for sale that you'd like to buy" would go without saying but I appreciate your advice. Thank you.

I was hoping someone else would give their opinion also, as it may be somewhat zone related as to how long it takes for a decent root system to be established...Ann

Thanks Ann. I think I'll play it safe this year and go with your advice on six weeks. I'll try to estimate how many plants I'll need at the market and then pot them five to six weeks ahead of each market date, instead of digging them all at once. It's a pretty small market and with this being my first year I'd rather end up the "extras" planted comfortably in their beds (where I have plenty of room) and not end up with a hundred or more pots on my patio over the winter. :-0 Thanks again.

For planned sales, I pot up even further ahead than that. For a series of traumas like that are involved in transport to the FM, time spent in the different environment of the FM, transport to new home, possible wait in yet another environment, and then the transplant shock, I would want the plant in the pot for a minimum of six weeks.

But my situation is different. I do have storage area, not unlimited, but sufficient. But in any case, I agree that a week is simply not enough time.

Ray