As far as buying a used semi-auto handgun:
-BE SURE YOU'RE GETTING FACTORY MAGS! Boy do people love to hose others with crappy aftermarket mags
-rifling of the bore...actually just check the bore in general
-top of the barrel where it makes contact with the slide as the slide cycles...locking the slide back will expose the top-portion of the barrel...this is probably the best indicator of the "age" of a semi-auto handgun
-ejector, extractor, feed ramp, and breech face...again, lock the slide back to check these out
-when working the slide, (hammer actions if applicable), and trigger actions, take particular note of any "grittiness" or any spring resistance which feels weak. Ask to see a new handgun if you're not sure what they should feel like.... even well-used handgun should retain 85-95% spring resistance of a new one
What you DON'T want to see is any surface which isn't smooth and flat. You might encounter wear to the finish, especially on the top of the barrel and around the chamber, but this happens after as little as 100 rounds through a handgun, so finish wear itself really doesn't tell you anything.
As far as buying a used handgun without the "original packaging," or in the XD's case, two mags, holster, case, etc. I would expect a serious discount for not getting those things.... you can get a new XD for $500 or less if you look hard enough.