Our experience with pre-purchases (or pre-orders) is that they tend to be ineffective unless it’s a heavily anticipated, heavily marketed title, and even then we encourage titles to have short pre-purchase periods on Steam.
For most games, we've seen much better success if you wait until release and run with a discount off your product during the first week of sales. This gives your title the biggest opportunity to create buzz and visibility at a point where customers can actually play the game and share their excitement with their friends. Customers playing your game also means those customers’ friends are seeing them launch the game and post screenshots, discuss in the forums, etc., which can drive those other users to check out the game themselves and take advantage of the limited-time discount.
Most pre-purchase campaigns just don’t bring in much in the way of sales, and it's far better--in terms of both revenue and customer awareness--to save that buzz and excitement for when customers can actually play your game or use your software.
Running a prepurchase/preorder can also be a risk for both customers and Valve.
For these reasons, we do not support running a pre-purchase except in a few rare cases with partners with which we have a well-established relationship and that have a proven track record on Steam.
Coming SoonWhat if we want to point customers to a Steam Store page prior to release?
You can put up a Coming Soon page for your product well in advance of your release date. This way, you can let customers know the title is coming to Steam and point them to the store page from other sites. This will also enable the Community Hub and allow you to start discussing your upcoming title with Steam users.
To learn more about posting your page as Coming Soon, please see Coming Soon