There are many football shirts for sale on ebay and within that selection are many fakes Now it is very difficult to spot a fake shirt on some listings, however there are a few things to take into consideration when bidding on these.
Firstly, where is the seller located, I would feel more comfortable buying a shirt from the UK as some (not all) shirts from abroad are poorer quality and not genuine. If they say best quality that would set alarm bells ringing - there should only be one standard on a geniune new football shirt.
Secondly, check out the buyers feedback, a higher feeback rating, with no comments on false authenticity is something to look out for.
Thirdly, check the close ups on the pictures - some sellers won't show close ups - how can you check what the shirt really looks like if they don't have these - they may have something to hide
Lastly, if you buy this season's shirt on ebay and are unsure about whether it is genuine, go into a sports shop that sells football shirts and check out what that looks like, so you can see if it varies from the one you purchased, for example checking out the mixture of the material that the shirt is made from. You can put the above measures in place and hopefully they should help to improve the chances of authentcity, so that you get a good deal. There are too many sellers selling fakes, not just football related, if people are more careful in avoiding fakes, then hopefully these sellers wont last on bay.
I hope this helps.