I like this idea of OP. Also...

Also yes. A reason I don't bother enhancing accessories and I only buy them from CM.

The suggestion doesn't account for the additional costs of down grades. To make it fair you would need like +50% crons 3x the base accessory amount you suggested

Unfortunately central marketplace also requires a stock and supply. Say for example, Not very many people are grinding or playing anymore or even selling a requested item at all.

There won't be a stock to buy in the first place. 

So I think things like this are like a failsafe in case no one sells a certain item.

If they had a system where if there is x amount of orders/demands on an item, PA or a bot can bi monthly or whenever they feel like it. Can sell x amount of number to fill some orders. At the end of the day, We all grinded and earned our silver, We should have it being spent, Not sitting on an order for eternity.

I thought about that, but I don't know what ratio add to make it fair, i would like to know the downgrade % rating at DUO/TRI/TET accesories.

What is the point of grind for accessories worth 80-250m since you need 60-100b per pen.