From your description of her life in the pet store, I assume she has the dreaded Metabolic Bone Disease. It means she is metabolically sick from the results of being undernourished + too little calcium in her diet + too little UVB. To maintain her nervous system in the face too little available calcium, her body robs her bones of the needed calcium.
You need to take her to a vet for x-rays. Depending on what's going on, she may be able to have her leg splinted so she can heal better.
If the vet can't stabilize the leg by splinting, ask him/her to show you the best position for the leg. Then, put her in a hospital cage that's so narrow she can't move forward or backward, or move much from left to right. i know this sounds horrible, but she needs to heal, so something like this has to be done. Otherwise she'll drag that leg around, and someday it might need to be amputated.
Maybe the vet can figure out a good way to tape her leg to her tail.