What about guaranteeing that the books on your website will be formatted properly, have been proofread and are are a pleasure to look at on a screen? Don't just leave it to the authors to do this job, a second pair of eyes always catches something the original writer may miss.
Perhaps, for a fee, you could offer editing facilities for prospective new authors? I've downloaded far too many $1.99 and $2.99 novels which look a mess and are almost unreadable.
I do realise Sturgeon's Law plays a large part in self-publishing!
