Stubhub has a pretty air-tight policy on fraudulent/counterfeit tickets. I, personally, have never heard of any fraud on Stubhub. If there was any issue, since Stubhub is such a large company and such a story would generate tremendously negative press, they'd likely be willing to work with you. If you're planning on buying tickets in the secondary market, Stubhub is probably the safest way to do it.
If you're still not convinced, look at the "FanProtect Guarantee"
FanProtectTM for Buyers
Summary of StubHub guarantee to Buyers:
You will get your tickets in time for the event
Your tickets will be valid for entry
Your tickets will be the same as those you ordered
If any of these things do not occur, we will find you comparable or better tickets to the event, or offer you a refund
http://www.stubhub.com/fanprotect-guarantee-legal/#buyers
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.