There are two stores: ebay and etsy. The ebay store is stronger since I've been selling there for a while and have almost 500 positive reviews. Etsy is easier to sell and I get a few pennies more per sale.
You can run the sales from your phone or from the web. It might be easier to start with the web - I was able to do video tutorials of the two stores - see below.
To mail, you put 30+ seeds in a pack - I usually put 35-40, more of the climbing milkweed because the seeds aren't the greatest. Then put in a little piece of bubble wrap - I cut up old bubble wrap that I get in packages. Stick them in an envelope with one stamp and ship them off.