The Secret Service says it's a nationwide issue -- people selling their older iPads, tablets, and other valuables on Craigslist are becoming easy targets for counterfeiters.
Those same people end up taking bogus $50 or 1$00 dollar bills and they don't even know it.
"The bad guys who are printing it here in the valley are not making a high quality counterfeit," says Kevin Rice, U.S. Secret Service.
But it's good enough to fool some people, even a student at ASU. Campus police say a student at the university's polytechnic school sold his old iPad recently, and the cash he took turned out to be bogus.
Kevin Rice is the special agent in charge of the Phoenix Secret Service Office.
"In the state of Arizona there is on average about $20,000 to $30,000 in counterfeit notes passed."
Rice says a recent rise in the amount of technology for sale is a popular draw for counterfeiters.
"There have been probably six or seven here in the valley in the last month," says Rice.
And Craigslist is a good place for the bad guys to start.
"It's a very anonymous transaction, you generally don't get to know the persons true name or true email address."
But counterfeiters can be foiled, if you know even a little bit about how they work. Rice showed us a counterfeit fifty. It started out as a five, but counterfeits bleach the bill, and then print a new one.
"In every note, except the one and two dollars, during the actual manufacture of the paper they put watermarks inside the paper."
In this case -- a fifty dollar bill has a five dollar watermark.
"This is a counterfeit fifty, its reporting to be a fifty, but it's actually a five, because if I hold it up to the light I see the watermark of Abraham Lincoln."
And according to Rice, if the counterfeiter thinks you are on to him -- chances are he will walk away.
"Most bad guys if they think you are an educated consumer and that you know the difference and that you are looking for those built in security features, they are going to look for a victim who is less educated."
You can learn more about watermarks and other security measures the government has added to most bills.