Before buying tickets: make sure there are no hidden charges (excess postage, booking fees). Also, make sure that there are no re-sale conditions attached to the tickets. Sometimes tickets that have been bought directly from a venue or accredited ticket agent are not eligible for resale and you may find that you are refused admission to the event.
It is always advisable to buy tickets from a seller who already has the tickets in hand and who is prepared to send them by registered or recorded delivery. Ask for a tracking number.
Check the seller's feedback.