Everyone has overlooked the number one tenet of dropshipping.
You must ship "blind", with no link back to you. You must fully understand that the people you are shipping to are not your customers and you have no right to solicit business from them - even 20 years from now. Without this basic trust you cannot be a dropshipper.
Like it or not, you will run out of stock. Every business in the world does occasionally. And I know of very few, if any, online sellers whose "stock on hand" amount shown on their websites is accurate.
Also to be considered is having sales materials for your sellers, zipped image packs, description files, etc. Possibly even RSS feeds.
The last is a "do as I say, not do as I do" sort of thing, but I'm getting there.