The client wants to list every single product on the homepage, separated by two tabs: "currently on sale" and "for sale soon". How do I tell him that a homepage is a gateway to other content, and its not a display case for all content available on the site? The current direction he wants to take seems to be to make the homepage a single page e-commerce website.
What's wrong with a single page e-commerce website? How do you know it's not an appropriate solution for the task? Focus on basics first.
You have a conflict: you want it one way, they want it another. It's a wash until one of you comes up with the research to back up your position, right?
Wrong. They're paying you (I assume) so the burden of proof is yours.
Dig into their motivations. Find out what job they expect their homepage to do. You mentioned a "display case" metaphor. Is that your characterization, or their words?
You also mentioned 2 tabs, "currently on sale" and "for sale soon." Why do they want that, have you asked? Is the site new, or has it been around a while? If it's an established site, monkey with the information architecture at your peril. Nothing's worse than moving the front door on users. You have to balance continuity with customer expecting against your designer's urge to innovate.
Think about your motives. What's your cause-and-effect hypothesis about your design? What makes it better? Does "better" have anything to do with their customers? Frame things in a way that you could put your assumptions to some kind of test.
And - at the end of the day, remember they sign the front of the check - you have to convince them, not the other way around.
As far as research is concerned - plenty of generous ideas already provided above. Start with your design theory as rationale, then maybe refer to some competitive examples. If you can't find anyone else doing what you're suggesting, good luck convincing your client to pay for the experiment.