Mike,
I did that from time to time, when we sold pens. But I had a slightly different perspective: If I sold the extra parts for conversion for $15 or more, I paid for the entire pen kit. So, the remainder of that kit became "spare parts". It never hurt to have extra "black plastic inserts" or pen clips--both of which broke from time to time.
Ed, I do the same, charge what a new kit would cost and most people have been happy with this price. Connie says I should add shipping to it but I never just order one kit as I figure that cost is covered.