Once you buy the fabric, it's yours. It would be unlawful to, say, copy the design of the fabric, or scan it and use multiple pics of it some way, or claim to be a rep of Michael Miller designs. In fact, I would not use or refer to the name Michael Miller in any of your marketing propoganda; plenty of people won't know who the heck he is anyway (I sure didn't).
The designer has basically released the item - in this case fabric - into the world to use. He's put a price on its use by selling it; if he's worried he's not getting enough for his efforts, he can increase his prices. He can't stop you from making bags out of it.
Once you take the product and make something else out of it, then it belongs to you.
Lots of things slow down in summer. People are spending money on their vacations and the local festivals (some buying crafts, no doubt!) and such.
Didn't Etsy have a marketing/advertising campaign a short time ago, which resulted in excitement and sales? Well, that campaign is over! Time for the next one. It's also up to you to market your site. Having a website doesn't guaranty any sales.
You could always check out your local community education program to find out if you can teach a class. You could provide a class through them, or through a community or neighborhood center. It can be an adult or a kids' class. Moms might look for something fun to do with little Billy or Suzy.
If you're art savvy you might work p/t at a local craft or art supply store.
You might approach daycare centers to see if they would offer a special class, featuring you.
Churches in my area won't offer anything that makes a profit (state law, apparently). Yours might be different.
Approach Red Hat Societies. They're always looking for something fun to do, and they'll pay you.
My favorite is to make a basic bread pudding, and bake in some inserted fresh raspberries and white chocolate chips. The mix up some ganache (equal parts of cream and white chocolate melted together) to drizzle over. Nummy!