I hate to throw cold water on this suggestion, but there are two problems with the idea.
Many of the pen manufacturers and distributors already provide this service of selling individual parts for their kits. The only problem is that the individual part often costs nearly the same as the entire kit.
Having a clearing house for pen parts is a good idea, but somebody has to pay for the cost and labor for space, sorting, cataloging, and maintaining an inventory list. The inventory alone could be several hundred parts for all of the kits and plating variations that are available. Add to that the cost and time required for receiving and answering inquiries, packaging and postage. I can't see these things as being something that happens for free. If it is done for free by somebody, they won't be donating their time and space for very long. In the end, the real cost associated with such an effort will be little different from that of buying the individual parts from the manufacturer or distributor, or purchaseing the whole kit to get a couple parts.
The other problem is that the most desireable parts would NOT be those that are in the inventory. The inventory would be saturated with those surplus parts that we are not using from the kits, and not be a source for those individual parts that we do use. As an example, I buy kits for the tubes, clips and nibs only. I don't use the center bands and finials for the clips and caps. From what I see in the galleries, there are a lot of other pen makers who don't use them either. Your clearing house whould soon become saturated with an inventory of those parts that we don't want, namely the center-bands and/or finials, and there is still no source for the clips that we do want other than from the manufacturer or by robbing from a kit.