Do I have a case if I broke my leg by trying out a used bicycle?

No. Used merchandise is sold "as is."

For the store to be liable, you'd need to be able to prove the store knew or should have known there was something wrong with the bike and it doesn't sound like you can do that.

I doubt it. Do you know how to ride a bike. A bike "jammed" what does that mean? If you contribute in any small way to your own injury you cannot recover in NC. How do you blame the store for your injury?

Michigan law requires that you prove negligence on the part of someone that you sue for negligence. The mere happening of an occurrence/injury does not give rise to liability. If you can prove that there was something dangerous/defective about the bicycle, and that the store either created the defect and/or had a policy of inspecting/repairing used bikes and offered them for sale as refurbished/in operating condition, then you may have a case. Generally you would need to have the bike examined by someone who regularly repairs/services bikes who could advise as to whether there was something wrong with the bike. You would then use their inspection/testimony to prove any theory of negligence. I am not sure if a reseller of used bikes is liable in warranty, and it would be advisable to consult a local Michigan lawyer and see if they can do some research into such a possible claim. Certainly if the shop indicated they had inspected/repaired the bike and were offering it as a working bike and/or offering a warranty, then they would have liability if you can establish that there was a defect in the bike (which, again, usually will require you to have an expert examine/test the bike). I suppose it is possible that your testimony alone could establish a defect and/or breach of warranty of fitness, but I wouldn't count on getting a settlement or winning a case on that basis alone.

It could go either way. You knew the bike was used and had rust on it. Hence, you should have been on notice of a possible malfunction.

There could be a case of liability, but it's a little weak: yes, they should have checked it out to make sure it was in good working order, but would a reasonable inspection have revealed whatever problem it was that caused it to jam? You knew it was a used bike and that it's condition was questionable, but you elected to test-ride it anyway. Would you have been able to detect the problem some other way? If so, why didn't you? If not, then how would you expect the store to find it? Find a lawyer in your area who is willing to take on challenging cases.

Maybe, but was there a defect in the bike? If not, it would be operator error. You need somebody to go back and take pictures of the bike, maybe even buy it, to see if there is a defect. If not, I don't think you have much of a case.

You probably have a good case. Send a demand letter to the owner of the thrift store and get the contact information of its insurance company. Make a claim with the insurance company and demand that the owner forward your letter. You need to keep track of all of your medical expenses and lost wages. Don't settle until you know the extent of your injuries. I recommend that you contact a personal injury attorney.

If the bike was defective, and you can prove it.