As stated above, we are a manufacture and a vendor of various weatherstripping and other restoration pieces. Taking your example above, most customers do not replace the vent window gaskets when replacing the door seals and roof rail seals even on a fairly involved restoration. So if we make a kit up with vent window gaskets included in the kit, the consumer will not purchase the kit as they do not wish to replace those pieces. The second area of concern is that manufactures will purchase from each other to offer coverage on areas that they do not manufacture. By having to purchase from a manufacture that is not able to discount to the level needed to maintain your profit margin, your not able to pass along any additional savings that people usually relate to purchasing a "Kit".
From being involved in the manufacturing and selling of various weatherstripping pieces for over 28 years, you develop a knowledge of which components work better for each particular car or year model. Depending upon the year model and body type, we make our suggestions based on parts from as many as five to seven different manufactures to give the customer what we feel to be the best parts at a reasonable price. That is the reason that if one particular manufacture makes up a kit, it will basically be just door, roof rail and trunk seals. But depending upon the extent of the restoration, a kit could include all of the inner fender seals, under the hood seals, vent window gaskets, quarter window gaskets, door end cap seals, U-Jamb seals, etc.
Hope that this helps to explain our view. I relate it to customers looking to buy interior kits. Everyone needs carpet and seat covers, but not everyone needs door panels or a head liner. Any other questions, just ask, I do not view it as being taken in the wrong context.
