You can't refuse legal tender as a business in the US. If you suspect counterfeit bills, you need proof (the marker or it lacks some security feature like water

You can't refuse legal tender as a business in the US. If you suspect counterfeit bills, you need proof (the marker or it lacks some security feature like watermark or strip) and then you're supposed to call the police immediately about it. You can refuse the sale in general... but that's a tricky one left to someone else to answer.