In Illinois, it seems to be legal at the present time. Connecticut supposedly has a law that forbids sales other than in a brick and mortar store, although that makes even less sense to me. If you want to gear up and look like the Michelin Tire logo ("Mr. Bibendum"), have fun. I think I'll enjoy the summer in shorts and a T shirt myself.
Please be sure to mark the best answer to your question. My answers are general and do not form an attorney-client relationship. I'm happy to talk to prospective clients in my areas of concentration and geographical location.
In Illinois you can legally wear/purchase a "bullet proof" vest...
If you live/work in an area with rampant violent crime, it may not be a bad idea!
If you do purchase a used "vest" be aware that the "Kevlar" shields have a shelf life of a certain number of years before the material starts to "break down" and have less stopping power for projectiles.
Sal Sheikh practices law in Illinois.This answer is provided solely for informational purposes only. You should always speak directly with a lawyer in your State. It is difficult to evaluate your legal problem without a consultation and review of all the facts and documents at issue. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.