It's a liability issue. I don't know if all of the retailers have a little disclaimer somewhere on their website, but I know that Glockmeister has on their packaging: "The determination of the fitness of the suitability of any product for specific use is the responsibility of the buyer. All firearms are potentially dangerous and can cause great harm, personal injury and death. It is your responsibility and legal obligation to handle, load and unload, use, transport, maintain, and store your firearm in a safe manner so as to prevent its accidental discharge. Likewise, parts for firearms are potentially dangerous and if installed improperly can cause your firearm to malfunction or accidentally discharge. These parts should only be installed by a Glock Certified Armorer. No liability is expressed or implied for damage or injury as a result of installation or use of this product. Warranty is limited to the replacement or repair of the original only (exception - stock parts)." Glock won't sell the parts directly to consumers because it'd be a liability nightmare. Even if they put a disclaimer some lawyers would try to argue that as the manufacturer selling these parts to consumers, even with the disclaimer, the manufacturer would be implying liability if the consumers detail strip their guns and improperly reassemble them. Besides that, there's still the possibility of a defective part that could cause dangerous malfunctions even if the pistol is assembled properly.