Honest answer is that you can't tell if a buyer is legit, but you are obligated to go through with the sale using the info that you are given. As long as you have documented proof that you transferred the tickets and confirmed the sale then you will get paid.
You should have asked your question before you canceled the sale. You made a mistake canceling the sale. You will be charged a 100% penalty for doing that. If your relisted tickets have not sold yet then remove the listing and call Stubhub or send a message to @TeamStubHub. Explain why you canceled the sale and see if you can get the sale reinstated and send the tickets. I don't think that you can do that after you already canceled but it's worth a shot to avoid the penalty charge.