A news report from News 3 LV gives notice that counterfeit money is said to be being used at Las Vegas yard sales. This likely isn't a surprise to people following news on the topic. We reported on our website a few years ago when a report came out that fake money was used at a lemonade stand which is similar to this story in how it could effect a small sale operation.
This article from Kansas says that the Wichita police have seen several casess like this in the past few months of fake money being used at garage sales. They also mention that you can buy a special pen to detect if the money is real or fake.
For anyone running a garage sale we'd recommend to check on money being received and look for any signs of it being suspect. U.S. dollars should generally have a similar feel to each other and the proper images on the bills corresponding to its denomination.
We would also recommend a counterfeit money detecting pen for selling items at a garage sale to check that money you receive is real and for bigger operations, counterfeit detecting machines may also be advisable. Feel free to check out our money detecting products on our site at iTestCash and be sure to follow the blog to keep up with news on funny money.