Well, here's what I did for a while with great success. Look in the newspaper ads each Sunday and look for all of the great "after rebate" deals in stores like Best Buy, Circuit City, CompUSA, etc, and buy the items if it's a really good price.
List them on eBay with a disclaimer that the UPC has been removed for rebate purposes, and there you go.
I did this with mp3 players and made about $500 a month extra back when I was doing it. This was before the iPod came out, which NEVER has rebates, so that market has changed a bit, but any well priced electronics item should work just fine.