Cannas would be borderline hardy to your zone 8 so it would be possible I assume to overwinter outside; probably mulching and planting in a protected area would increase chances of success. If the fellow at the farmers market said he has had success with this I would hope he is honest about it but of course factors in your own yard would play a part. Why not try dividing your plants and plant a division outside as an experiment? I'm not aware that there are cannas that are more hardy then others but that is a possibility.

The plant you brought in last year, did you continue to grow it indoors or just store the rhizomes for the winter? This is what most of us cold winter gardeners do. Good luck!

The one I brought in last year, I did try to keep it growing, but it died down mostly. Just a couple small leaves were left come spring. But it came back bigger and better :) I didn't even know that I could just store the rhizomes until just this week after doing some research. lol

Yes that is what I will do, I will divide the one that I just bought and plant some in the ground to see what happens.

Is it better to let the Canna die down and store the rhizome? More resting time? Or is it ok to keep it growing?

I don't know if either is better but depending on the variety of canna it could get pretty large to overwinter indoors if you're going to keep growing it. In my case I grow all my cannas in pots. Once the frost hits them outdoors and blackens the foliage, I cut off the foliage, lug the pots inside and store them in the basement in my coolest room. In the spring I repot with new soil and start them growing again. Quite often they will put out some weak growth over the winter just from the heat of the house and I will cut that off. Or you can dig up the rhizomes in the fall and store them over winter in peat moss or the like and them pot them up in spring. One year I just threw the clumps in a plastic garbage bag and that worked okay too.I find its best to keep the rhizomes in a clump so they don't dry out and divide in the spring. They do multiply so you will get more plants. Of course, your season is a lot longer and milder than mine so that may have a bearing on how you want to treat them.

Drainage is critical for any plants for which one is on the fringe of hardiness. Keep adding organic matter to the soil surface to improve that. When dormant for winter, covering marginally hardy plants with copious amounts of leaves can also help with zone pushing/cheating.