I concur with Bruno.
However, I have some questions.
Let me preface it by saying that I have attempted to find specifics on the site, and did not find enough to warrant a purchase.
That said, how much of the traffic on your site is the same as the traffic on TCDB? If it's 60%-75% (or higher), you'd be better served to port everything here (and then direct the other percentage that you get from elsewhere to here).
How hard is it to get organized? I realize you have millions of cards, and I'm talking a smaller scale when I dealt with the 35-40K I had from my best friend when he moved. It contained baseball, basketball, hockey, football, and some mixed non-sport and multi-sport releases. Using a couple extra boxes to begin with, I sorted them out by sport, then once they were together, I sorted them out by year/set. When I didn't have the room to pull everything out, I sorted what I could, boxed it (with some papers inbetween to help with year identification), and filled in when the next sort happened. Again, I know this was only 40K cards, about 1/12th what you have, however, it was still an operation that was scalable. (In fact, the first sorts by sport remained in each of the boxes they were already in, just finally separated.)
Is this your business or a side project? Either way, if you're serious about moving product, get serious about it. If it's a side project, shut things down for a month or so and get it organized where you can bang out a ton of stuff. If it's your business, you may need to spend every waking hour to get your product seen.
Do you have an "outlet" for the "overstock" product you have? If you see that you have a glut of 1988 Donruss and Topps (or the like), and you have no need to list more than 10 (or 20) here, but you have 80 of certain cards, can you move them in bulk in another method? If you were able to reduce your "unmovable" stock, you'd have better access to your stock that sells. (Or store the overstock off to the side until the "normal amounts" start selling, then start bringing them into the regular stock - if you haven't shed it all.)
Again, I acknowledge that I don't have experience with an inventory the same size as yours. And my personal inventory was only one sport when it was closing in on that number, so I may not fully grasp what you are facing, but I do know that without some sort of change in your efforts, nothing will change.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).