Counterfeit money is a bit risk when doing this. There have been several stories of localbitcoin sellers receiving counterfeit money.
Also, on $100 bills, Ben Franklin has a ghost image that can be seen holding up to the light.
But better than all of that is to require the transaction to happen in a branch of your local bank, or post office, and be funded by a money order from the bank or post office.
Some people have visual impairments but they really shouldn't be dealing with large volumes of cash.