Flat fee to the publisher (if they hold the rights), advance and residuals to the author? Get the authors to write a blurb about themselves on your website as a value-add and use a commission system to pay them, maybe? There's software out there now that will handle that. You want the bookkeeping to be as automated as possible.
I'd suggest looking at a web CMS like Drupal (
http://drupal.org). I think it may have everything you need from the software side.
The harder part is likely to be tracking down ownership of the ebook rights per title, especially when the author is deceased and/or the publisher has gone out of business.
Even with that hurdle, you'll soon have more titles than you can process immediately. One form of prioritization that you might use is customer requests, of course, but another is cross-checking these requsts against used book prices -- if the used book price is high, you've probably got a good market.