I used to buy marigold seeds every year and also zinnia seeds as well as viola. The viola, if left alone to freeze, I have found, often come back. But as for the Zinnia and Marigold seeds, I let the flower get dry, pluck it off of the stem and put them on a paper towel until completely dry. Then I take them and open them and loosen the seeds into a paper bag. I place that in a container where nothing can get into it and the next year I plant those seeds or at least some. I went from four Zinnia plants my first year to 70 the next and I went from 4 Marigold plants to so many seeds I gave some away because I had so many I could have planted about 4 or 500 plants. Yes, it works and instead of paying 1.00 for those 10 or 12 little seeds, I have an unending supply. AND, they get prettier each year!
I said the pic of my marigolds was 7th generation but it was last year so 6th. This year will be seven. These are the zinnias from last year. some are full bloom and some are not.
Here they are from last year. Just starting to pop up this year! the other variety I have are even more beautiful and very full bloom!