I have some experience with buying ashes tickets from ebay for friends and also selling the ones i bought throught the proper channels that I didnt want for various reasons. Anyway here are some pointers...
1. Never bid on tickets where the description doesnt state details like, where the seats are in areas of the ground, whats date its on etc etc. If you want tickets firstly watch the item in your ebay and then contact the seller. If no reply do not bid on them tickets.
2. Also be aware of tickets that seem to be well under what they are worth. This can often, not always. But, often lead to you not receiving tickets. ie... I spotted sydney tickets. Apair for all 5 days and all he was asking for was next to nothing.
3. Also, report any dodgy emails. ie... the ebayer who had bid on these sydney tickets told me after I had warned him to check out the seller, that the seller had refused to sell the tickets he had successfully won for anything under £300. Around 4 times as much as they went for.
4. Also, I spotted a rat when these tickets with exactly same photo were appearing over and over again on ebay. This seller was using 1 day auctions and getting several sold. I suspected the seller had stolen the photo or only had 1 set. My suspicions were confirmed after I told ebay of this and other tickets I had bid for that didnt have any details or any photo and appeared over and over again. I had agreed to buy a set of several tickets for £12! Anyway, ebay has confirmed this seller was dodgy.
5. In my opinion all good ebays that are selling items will reply to emails pretty quick. If the items sells without you having heard from them then that is a sure sign that this may be dodgy.
6. Finally, never buy tickets where seller has revealed seat numbers, that way you wont have them cancelled.