The pens are worthless in general. Real bills can "fail", fake bills easily pass if they care to. The Secret Service will tell you to not rely on them if you call them up and ask about them.

A friend of mine had a $10 bill checked and confiscated at a corner store a few years ago. The bill was genuine, but a few decades old and over those past decades it accumulated enough starch on it to trip the pen. He had to call the local SS field office and get them to talk the corner store into giving him back his bill (the SS looked up the serial number, said that it wasn't a serial number known to be used on counterfeits, told them that the pens were shit, and told them to give the bill back).

I say the pens are worthless because, although they can detect counterfeits that somebody ran through their ink-jet at home, those sort of counterfeits won't fool a standard "look and feel" check anyway. In fact, they may give you a false sense of confidence and distract you from the more robust "look and feel" check.

I was responding to a specific point grecy made. People will check a $20. I imagine they might even change their method of checking if a different type of counterfeit becomes popular. I've only ever seen the marker or a UV light used in public, but criminals aren't the only ones capable of adapting to new circumstances.

Hopefully it's obvious that the seller mentioned in the article is probably not making or selling a darn thing. What authority are you going to go to when you find out the counterfeit currency you bought online anonymously with bitcoins was just a scam? His scam is, on a small scale, more profitable than actually manufacturing counterfeit bills and selling them.

Which doesn't mean the seller won't get a visit from the US secret service if he's crazy enough to be running this scam outside of Russia or Nigeria or something...