The biggest market is for Windows-compatible PC Accessories. Check the descriptions to confirm compatibility with Vista. Many computer accessory products are also Macintosh / Linux compatible - open up the descriptions to check.
If you are interested in selling computer accessories, you should also check out
USB Bluetooth Adapters,
Cool Gadgets, and
HDD Storage Enclosures.
Tip for importers: many computer accessories are small and cheap when sourced from wholesalers... the best way to profit is ordering in bulk to minimize per-piece shipping costs, and keeping your own stock to send out to customers.
Editor's Note: If you're not working entirely on your computer, you will be soon. So accept the inevitable and gadgetize your PC / laptop! Geek essentials (speaking for myself) include fingerprint security to log into Windows, wireless keyboard and mouse, high definition conference microphone for Skype calls, and several USB toys like the Plasma Ball and USB Shredder.