In the most recent builds we got some more clarification on how the upgrade process works for Legendaries and we know how a pretty good idea of how this is going to look.
The base components
To craft a legendary you need several different base components. You need a base item that determines the item level. These are crafted by all the various armor professions. In addition you need missives crafted by inscription that you use to determine the secondary stats of the item. Lastly you need the legendary power and Soul Ash from Torghast.
Upgrading the item level
You can also upgrade the item level of an already completed item. To do that you need the Soul Ash difference between the item level you have and the item level you plan to upgrade to (100 for each tier). In addition you need a new base item of the higher item level.
The consequences for gearing
Soul Ash is capped per week at 100. This means that to be optimal at all times you will want to craft your first legendary at the lowest item level during week 1. Then you will want to upgrade it every week to always have the strongest legendary you can. If you plan to multispec you may want to craft another level 1 legendary for that spec.
Profiting from this
Base items should be very in demand for a while. For the first week you will want to focus on the necessary progression. Rank 1 base items week 1, then rank 1 and 2 items week 2 etc. The missives will mostly sell when players craft a new legendary, so they should be particularly strong in the first week and being first to market with the most important stats will be huge.
Overall it seems like demand should be really strong for many of these items and I strongly suggest getting the necessary professions to get going ASAP.