Occasionally, the app offers Star Wars Rebels card packs for purchase, with credits. These card packs include Star Wars Rebels insert cards, that are only available in card packs for a limited time and in very limited quantities.
I have been able to "pull" a couple of these inserts, and I have been able to trade for a couple other Rebels cards, with other collectors using the app. If you are using the app, search for me JimmyinGA and offer a trade.
The official description of the app from StarWars.com is listed below.
Collect and trade over 1,000 officially-licensed Star Wars digital cards. All of your favorite characters, vehicles, and locations from the Star Wars universe are here. From the nostalgic, including replicas of the original “Star Wars ‘77” set by Topps, to the futuristic, including all new trading cards with exciting digital twists, you’ll be able to take your entire Star Wars card collection with you anywhere you go. Star Wars™: Card Trader…these are the cards you’re looking for. Available On: iPad®, iPhone®, iPod touch®