The pens work but the tip is hard and you have to scribble a couple of lines in order to see where the marks are. You would be better off to get a pen with a tip like a felt tip marker.
Now a days one has to be careful when selling items from your home. People paying with cash you can never know if you will get burned with a counterfeit bill. So I use these pens when I yardsale and mark every bill $20 and larger since that is what the banks and credit unions I have spoken with have told me the smaller denominations (ones fives and tens) are not worth the trouble; or so the story goes. The pens work great though!