Always check sellers feedback and that it refers to the sale of tickets.
Always ask to see the confirmation of purchase email before paying
Always keep in contact with your seller. Make sure you have their email address and not just their ebay username. A good seller will keep you informed.
Always ask for Next Day Special Delivery. I know this to my cost. I used to send tickets by Recorded Delivery but this is not insured and so seems to highlight the contents as valuable but not particularly important to Royal Mail. Suffice to say that I lost 3 seperate deliveries via Recorded Delivery ( which were either replaced or refunded) and I have never had a Next Day Special Delivery item that did not arrive before 1pm the following day.
Be sure that you know exactly what section of the venue the tickets give you access to. Front standing means exactly the same as standing. If the tickets are seated, check the venues seating plan ( just type the relevant venue into google) do not always beleive the sellers plan. It is not difficult to doctor them.
If a listing tells you a gig is sold out make sure that it is. You should only go to ebay if you have first checked (and rechecked if some time has elapsed) the original ticket agencies or if you are looking for premium tickets i.e. front row seats.