I have sold several high-value cameras on Craigslist. I am in the midwest, so ymmv in other areas.
I strongly agree with the idea of meeting the seller at the purchaser's bank. He knows that you are giving him real money, and it is a safe place where you can feel relatively comfortable, since it is your own bank.
I don't believe that you have to pay sales tax a second time if the purchaser already paid sales tax when it purchased the camera. As the 2nd buyer, it isn't your obligation to ask if they paid sales tax already, so if they don't mention it, you can assume you are safe.
However, I always lose a proportion of my purchase price, so it would not be legit to buy a 5D3 for $3,500. $3,100 would be the price I would expect to sell one for even if it was still brand new.