I'm interested to hear what people with actual credentials say about the historiography of slavery.

Yes, this is a necessary step of any historical research project! Ideally, a major history paper (although I don't know about the specifics of your particular assignment) includes a section at the beginning that discusses what earlier scholarship has said about the topic of your paper, or about things related to your topic. So you might have a section about "what scholars think about Europeans describing the early modern Atlantic slave trade" and one about "what scholars think about Arab Muslims describing the medieval/early modern trans-Saharan slave trade"--unless you can find a good amount of comparative work. (Which--maybe that is out there; you'd have to find it).

To find previous works of scholarship with "actual credentials"--that is, books and articles that have gone through a peer review process to ensure the authors have some basis in source material and have put forth a logical argument based on them--you would use your school library and online databases/search engines like Google Scholar, JSTOR, ProQuest, depending on what your school has access to. (JSTOR seems to be pretty standard in the U.S. now, but I don't know about elsewhere). Then you would read the books and articles that look promising.

What databases and searches have you tried, that haven't been productive for you yet?

Great project! It's an area where some groundwork – of varying quality – has been laid, but where more could certainly be done. Regarding the slave trade itself, I've mentioned it briefly here and here. The latter comment might give you some wider temporal and geographic context for thinking about the Trans-Saharan slave trade, and the Haour article is simply indispensable for your idea.

But it sounds to me like you're actually asking a more interesting question than previous scholarship has pointed toward. Rather than simply asking about the trade itself, you're asking a comparative question about how it's represented in writing. (At least that's how I read your question.) There's a massive amount of material on the Atlantic slave trade, so your best bet is starting with the Trans-Saharan trade and looking at what kind of analysis the source evidence will support.

For starters, I'd dig through Haour's footnotes and an older article she cites: Fage JD 1969 Slavery and the Slave Trade in the Context of West African History, Journal of African History (10): 393-404. Alternately, you could take Ibn Khaldun (d.1406) as your starting point. He was a North African historian, and if you find an online edition of his monumental and still widely respected Muqaddimah you can do a quick word search for "slave" (it turns up 38 hits; check out the entry on Ibn Khaldun in 50 Key Thinkers on History for a quick assessment of his work). A similar source, moving a bit later and into the period of the Atlantic slave trade, was written by Leo Africanus (d.1554).

The more I think about it, the more I could keep spitballing, but that'd spoil the fun of finding your own project. Lots out there; lots that can be done. You've got a great idea going, and I think it'll be tough to keep it down to 2500 words.