John, something in your question makes me think you see something deeper that others have not thought about.
To me, it comes down to - Where is the profit? Is there enough profit?
On the pure surface of your question, I doubt there is a worthwhile market for a pre-turned specific kit. There will be a few people (entrepreneurs) who do not have the wherewithal to turn to fit, but want to buy and assemble and sell for profitable trade. But these guys will want quality work for wholesale prices. I doubt many people will do this. I do know that I have seen many sellers at some crafts fairs that have low level quality and would be glad to do that but the buyer (of the pre-turned to specific kits) - would totally reject that. For high quality fit and finish, I doubt that many of these experienced fellows/ladies would sell low enough for the assembler/seller to want to buy.
There was a post here about a year or so ago in which the guy wrote something along this line: "Who better to sell finished pens to than pen turners!" After his post, there was not much discussion along those lines. 1. By the nature of what we do, 2. by the sheer numbers of those viewing these pages daily, and 3. by the sheer range of experienced turners, in my opinion, there will be very little sales of pre-turned blanks for specific kits, particularly turned for precise fit. IF one is not experienced for excellent fit and finish, their product will not be wanted. For the experienced excellent fit and finish maker, I doubt they would not sell at the low price that an assembler-seller would want to buy, except on a very specific one-off sale.
Rare One-off sales, yes.
There is already an outlet that many have used and still use to sell pre-turned and pre-finished blanks to fit specific kits (and assembled, which takes 10 to 15 minutes) and that is consignment stores. I separate that from individuals who do direct sales. A consignment store is where pre-turned and pre-assembled kits/component pens are sold. And consignment stores charge from 20% and up. IMO, not many will turn for that much holdback on their work.
Let's just say:
A kit costs $25.00
A finely crafted segmented or other blank is turned and fit and finish excellent. ($20 components for blank)
Sale the pen: $150.00
Raw profit: $105
Time, experience and investment in equipment and consumables: $60.00 ( + or -, depending)
(An experienced and artistic turner with precise fit, finish and artistic value will not turn for less than $40 - $60 an hour, at least I will not.)
Net Profit: $45.00
A buyer of fit and finish blank would want to buy it for $40 - $60 at the most.
So, If I do purchase the raw blank, make the segments or cast the blank to specific request and turn and finish to required tolerances, that would be an estimate of $20 + $60 or $80. The buyer wants to pay $40-60. Not worth MY time and I am not in the same league as you are John. Your experience, investment in time and equipment would double the price that I mentioned. You are an artist with rare eye-hand coordination for exquisite creations.
(I speak from living and being around renown artists in Japan and in and among generational poverty artists in America.)
Net profit from sales would be negligible at best, but probably at a loss.
To me, it comes down to - Where is the profit? Is there enough profit?