Compare the specifications in their patent against puck holders you sell. If the ones you sell would be covered by the specifications (called "claims") in the patent, they are infringing; if they infringe, you can forced to stop selling them (to stop infringing) and could be sued if you don't stop. On the other hand, if the holder you sell does not fall under or is not covered by the patent's specifications, there is no infringement and you can sell them. (Though they may still try to sue you, forcing you to respond to a lawsuit, if they disagree and believe the holders are covered by their patent.) Intrepreting patents can be technical: you may wish to hire a patent lawyer to help you, though be warned--such lawyers can be very expensive.