I'm not saying this to the OP, but it is a general statement.
Always pay with CREDIT CARD, so that you don't end up having to be burned. In this day and age, where tons of even little ma and pa businesses can process credit card sales, there just really is no reason to pay cash to any kind of business where you would assume risk. If you buy a hamburger you aren't assuming much risk. If you buy a product that costs real money and won't know if it is in good working order until later, insist on using CC.
Dealings with other private people is a different matter and of course usually they can't process a CC. Even then, checks and or bills of sale are wise. There is no proof of payment with cash alone. None.