Go for it!
If you want to give out your decks for free as samples, as promotional materials, or just out of the goodness of your heart, you can post them for 0 points. If you do so, anyone with a Boom Learning account can “buy” and play those decks without paying anything. Any person buying such a deck gets to keep it forever, even if you later raise the price. Sellers are encouraged to make some of their materials free. People are much more likely to look at your store if they sample free products from you and like them.
If you want to have a short term sale, announce the terms of the sale using whatever means you use to reach your buyers (blog, social media, etc.). You will need to manually open the product and change the price in the details in time for sale launch and change it back at sale end. We do not at this time have a way to show the prior prior, so you'll need to announce the percentage off elsewhere.