As everyone has pointed out, do your due diligence. Understand your margins and as rockwell pointed out, use as many direct sources as you can. Sometimes if you buy in bulk they'll discount your shipping. As long as you know your numbers, you can determine whether it will work out for you.
Keep in mind, too, that not everyone will be honest in their listings, or might not know everything that's wrong with a device. You might have situations come up where it's nearly impossible to profit off a device. If you already know enough about it, or put in the time to learn, you might be able to use those as part sources.