I think she rented the dolly to take her own car back. She was going to drive the UHaul and needed to tow her Infiniti.
Ding ding ding.....
JA testified in guilt phase that Travis sold her the car-- she made it sound like she had done him a favor-- and that it was a crappy car.
So you know that the BMW had value, especially to Travis (that's why she trashed it) and that he never sold it to her. I don't know what she told Travis about needing his car to go get the UHaul...or maybe she just took it without his permission.