Hi. I'm sorry for your losses.
It's most likely a coincidence that the bunnies both died within a week of your bringing them home. Barring some of the possibilities that Roberta and Jerseygirl mentioned regarding a toxin in your home environment, my inclination would be to say that the bunnies might've had some illness they picked up from their time at the breeder, or the pet store; maybe e. cuniculi. It's an illness (caused by a protozoan parasite) that can present itself in just the ways you described: It can cause a bunny to not be able to stand up - either because its hind legs are partially or fully paralyzed, or because the bunny is too dizzy to stand; it can cause the bunny's head to tilt slightly or very noticeably; and it can cause tremors or convulsions, as well as other symptoms. The thing about e.c. is that it can have terrible consequences, or none at all, depending on how strong the bunny's immune system is and where the protozoa is located in the bunny's body. If it winds up in the bunny's brain, kidneys, or eyes, and if the bunny is under stress from any thing (like being in a new environment, or having surgery, etc.), then e.c. is more likely to cause illness. This could explain why the other bunnies are fine - if they were exposed to something like e.c., it might never make them sick, or it might make them sick years down the road. The cysts can be dormant in a bun's body for years. Just thought I'd toss this idea out there; my sweet bunny, who came from a pet store, has e.c., and it stayed dormant in her little body for almost a year after she'd had any contact with other rabbits. (E.c. is spread when a bunny inhales or ingests the spores shed in the urine of an infected bunny.) My bun, who is my only bun, had to have picked it up from her mama, or from the other bunnies she was with prior to coming home with me. There are other illnesses that can cause symptoms like your bunnies had; one is caused by a type of roundworm. Like Sarita and RabbitPam said, with so many rabbits coming and going from the breeder and the store, there's just a lot of potential for illnesses and less-than-cleanly conditions. Your use of bleach to disinfect the cage was good; you can make a spray using two tsps of bleach to a liter of water to kill parasites, bacteria, etc. For regular upkeep, use vinegar and water.
I'm sorry this is so long. I hope it helps you feel a little better, though. I know it had to have been hard to lose those little sweeties. Please consider adopting as the others have mentioned; it might save you some heartache.