Well, from a new consumer's perspective, I can say that two things really drew me to RPGNow: the free Tri-stat DX game and Monte Cook's Unearthed Arcana excerpts. By no means am I new to PDF gaming products in general. In fact, I have a rather large collection of them, starting with Hero Games' various books that I bought, to the things I have found online, here there and yon.
Someone suggested that you should go to publishers with a list of PDFs you found on Grokster/Kazaa/the binary newsgroups and encourage the publishers to let you sell their product. I would like to suggest the opposite -- finding a way to reach out to the people who are downloading the books from those sites. Because I can tell you from personal experience that a vast majority of the material that is available through the pirate channels is very poorly scanned. And they rarely have been rendered in OCR, so the inherent benefit of searching a computerized document is eliminated. I think a case could be made that people get what they pay for and for high-quality, OCR books, your site is the way to go.
So, maybe intentionally floating a PDF catalog on some of these sites/newsgroups would be a good first step.
One obstacle to PDF sales that I haven't seen discussed so far, and I think it is a big one, is the relative usefulness of a PDF gaming book. Frankly, it is not always convenient to have only a PDF copy of a book. And once you print it out (unless you are using a work printer, or something of the sort, where there is no cost to the user) the cost is now comparable to an actual printed book. For example, if I buy a 200-page PDF for $5-$10, then print it out for 10 cents a page, then put it in a $2-$3 binder, or pay the same to have it wire-bound, that is nearly a $30 investment. And if it is a color book, well there is no way that it can be printed at home with the quality and cost-effectiveness of buying the hardbound version at the game store.
And here is my final PDF thought, assuming that most people are choosing NOT to print their PDF out and incur that expense, instead choosing to read it on the computer screen, why are PDF documents invariably formatted in the portrait style? This requires the reader to scan down one column on the screen, then backtrack up the screen again to read the next column, then page down for the next page. If PDFs were formatted for the computer screen, i.e., landscape, the person reading at the computer would have an easier time reading them, while the person printing them out could simply turn the printout sideways for reading.
Just my thoughts.