If you are digging in mid March in Canada, you are going to have some pretty sorry looking rhizomes for sale in May/June. That is because they have not had a chance to start growing this season, and by digging and storing them, they won't get the normal spring growth spurt either.
That is not to say the rhizomes will be dead; they just won't look that good and their chances of survival are less than rhizomes which have had a chance to grow some in the Spring. If you decide to do this anyways, they should be planted immediately in May/June; don't wait for Fall planting: the poor things won't be able to establish themselves before the winter.
Laetitia

We are a Zone 7, so they may get some growth in this spring, but regardless, they're garbage if we don't rescue them.

Would they be happier if we pot them when we dig them instead of storing them?

Potting them would enable them to get some spring growth and they would look better at the sale. Depending on how many you have, I don't know if it would be too difficult to pot them all up. If you don't pot them, they won't die, they just won't look so great. Still...if people understand they are 'dormant' and will regrow once planted, that would work.
Just remember: if you do not pot them make sure you store rhizomes in a place as dry as possible, with air circulation. Do not store in a plastic bag (paper is OK). You don't want to get any mold or rot on them.
Laetitia

Thanks so much Laetitia.

I live in western Washington, and I have dug up clumps with as much dirt as can be lifted with the clump and simply planted the whole clump in the new place.
You can dig them up in July/August and divide then. A good handful of Super Phosphate mixed in the soil around them would be benifical also. They may even bloom this spring, that would be a bonus.
Rod

Thanks Rod. I'll pot them in reasonable size clumps and sell them that way. (with directions on how to divide later in the summer)

Another thing you might think about if you have the space for it would be to just heel them in somewhere. Just make a pile of dirt or sand and plant them all in there. Since you aren't growing them to bloom this year anyway they would do some growing, you wouldn't have to buy a lot of pots and you could just get them right out with better roots and more growth and sell them as loose rhizomes. I live in zone 7a and I've done this with lots of plants. I have a large sandpile where we had a pool. Good luck with the irises. It's nice of your friend to donate them to you.

Nancy