. "I guess my next step will be planting it outside but I am just confused why the vendor would have sold it as an indoor plant if it was supposed to be outside."
Because he/she was out to make a quick buck and didn't really care about what you did with the plant after they sold it to you. Small juniper bonsai are produced in the hundreds of thousands in China and other places and exported for wholesale in other countries. Additionally, many nurseries in the U.S. and Europe produce thousands of them too.
The plant costs about $1 or less, the pot the same in mass production. The mark up is well over 100% at retail. If the plant dies after you buy it, you might even be tempted to buy another at those inflated prices.
Before you go any further, do some research. There are many kinds of plants that can be used as bonsai. Some are best kept outdoors, others can tolerate indoor care (primarily tropical plants like ficus and schefflera.)
There are many places on the Internet you can get one, as well as places "in real life" too. The vendor sponsors here on this site can be a good place to start. Also you might look for a local club who can point you to local reputable vendors - Start here:
http://www.absbonsai.org/USAClubs.html