Buying sporting event tickets from e-bay/craigslist...?

I want to purchase NFL tickets for a family member for a Christmas gift. I have found that the cheaper tickets are located on sites such as Craigslist or EBay (where there are no fees, taxes, or shipping). How trustworthy is this? Most of them want to e-mail the tickets rather than send hard copies and they all want to use paypal. I am a bit weary of this; how do I know if the tickets are real? Anyone with experience know a little more than I do?