Thanks all for the feedback thus far. Have had some replies on F4G also that welcome the idea.
Let me ellaborate a little bit.
As Tabbitha mentioned it would be a guild of crafters. Crafters only too. So 'protection' moving to and fro would have to be hired. The hosting town would also be expected to supply some guards to keep law and order in exchange for a percentage of the markets takings.
I would hope to be booked to do a market by towns rather than have a planned route so as not to get stuck in a repetitive cycle. If towns wish to book a recurring date then that is different. Adverts will be placed in the usual places etc. Plus it would be in the towns interest to do a little advertising as well as the market themselves. The market could be booked in conjunction with another event like a joust for example. Each event complementing the other.
The market would be a place to both buy and sell. As crafters I think it would be a little more IC to buy in resources from suppliers. Maybe even trade items rather than use coin in some instances. If we had enough stock we could sell on also. Even possibly opening up trade between some parties that do not see eye to eye, with the market as a third party. Or all is hush hush.
For crafters already guilded then 'guest' stalls could be set up. I certainly would not want to exclude folks that wish to take part.
As for timing I think two weeks apart is a minimum. To arrange trade agreements, bookings and suppliers I would like to do IC as much as possible. Some of it will end up OOC here and there I'm sure.

The organising could keep members quite busy. For others it would be a few days here and there for the actual market days and some preperation. Depends how 'hands on' people want to get I guess.
Think that covers most of it. Keep the thoughts and suggestions coming please.