I don't collect manuals specifically, but if I see a manual for a game I own at a good price I'll usually pick it up. Like just last weekend the local hobby shop had one for Centipede and I grabbed it for a buck. And I always check for a manual when I buy used "disc format" games. For older cartridge based games, I have acquired quite a collection of digitally scanned manuals over the years, since finding the original manuals are so difficult. If you're ever so motivated, it'd be amazing to have a scanned archive of your collection similar to what member TSOLFan did over at ratigators.com.
I've had awesome luck buying item lots on eBay over the years. It's a great way to get good deals, and if you're willing to resell the unwanted items it can even be profitable. Are manual repros common on eBay? I'd imagine that could become a problem if unscrupulous sellers start misrepresenting them as originals.
