One easy way is to use a Counterfeit Detector Pen which interacts with the paper. It remains yellow if real,but turns a dark blue or brown if fake. This can easily stop the unsophisticated fraudster who prints fake bills from their home computer. This is the method I recommend for most sellers who do a "Meet-up" with the buyer to exchange cash for the item. However, it may not work on a more sophisticatedly produced, phony bill. 

If the item being sold is expensive (several hundred dollars), consider meeting the buyer at his/her bank. Observe the teller handing the buyer their cash and they then turn and hand it directly to you.

Below are Kevin's recommended research sites to help you determine if the Cash being offered is real or counterfeit.

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