Take it back to the store - that tree was gone long before you bought it, probably watered every day along with all the house plants. Junipers like to dry out almost completely before being watered again, plus they have to live their lives outdoors, and can't live inside for long, for many reasons. The ponytail also can get quite dry between waterings, and needs lots of light, but outdoors only in the south. Get some books on bonsai and read up on general care, and which trees are good for what conditions, then practice on inexpensive nursery stock.. not cuttings stuck in a pot and called 'bonsai'.

I am going to disagree w/Lucy on this one. Vince your post is not very clear (but when I read it)it sounds like you had some Bonsai stock and then were very eager to repot it into a Bonsai pot. I remember when I had begun the hobby I wanted to put stuff in pots right away, but as you learn you will learn that many things you grow may take years for you to put it in a bot. I basically had to learn that plant material had to be worthy of a Bonsai pot.

When you repotted did you use a rooting hormone to expidite the root regeneration? After the shock of the new enviornment(from the store to your home) and the shock of being root pruned did set it somewhere where it could rest and recooperate? Or did you place it in full sun? Did you watch the watering? You can't water a tree like normal after you root prune. It doesn't have enough roots to take up the water given. Maybe it was too little watering and the roots dried up? Did you trim foliage in proportion to the amount of roots you removed? If it had the same amount of foliage and less roots water evaporated faster than the roots could supply it. Kind of hard to tell you what specifically happened to the juniper without really being there.

Try again with another juniper from a garden center, but this time don't root prune. Learn how to keep it alive for a couple of growing seasons and work on the stlying. Lets say you get a juniper in a 15 gallon container, You may have to prune some roots over the course of a few growing seasons before you can get the root mass small enough to fit into a pot.

Conventional wisdom tells us that you can't grow a Juniper indoors (not saying you did this) but when I worked in a Bonsai Nursery I encountered a Juniper that started life as a 'mallsai' and had been treated as a house plant for around 20 years by a then really old lady. The darn thing was huge and grew a lot during that time. It doesn't sound like it should happen but some people have really green thumbs and no matter what they do wrong they can't kill a plant.

Then there are people like me, who are all Brown Thumbs and over love their plants.

Vince,
I agree with Lucy on this one. The juniper was in a bad way when you got it. Please do not discount Mr. BT's advice. The info provided is very relevant.
Vernon