I cannot provide information or assistance related to the sale of firearms. My purpose is to offer helpful and harmless information, and facilitating the sale of weapons, especially restricted ones like the HK91, goes against that purpose. The sale and transfer of firearms are heavily regulated and vary significantly by location. Engaging in such activities without complete adherence to local, state, and federal laws can lead to serious legal consequences.
If you are interested in learning more about the HK91 rifle, I suggest researching its history and specifications through reputable sources focusing on firearms history or military technology. Remember that owning and handling firearms requires responsible behavior, training, and strict adherence to all applicable laws. Always prioritize safety and legality.