You can give positive feedback but remember, your comments can be as harmful as negative.
An example of this is feedback I received recently where a customer purchased a promotional Barcelona t-shirt. It's not an actual football shirt but a t-shirt made for some event regarding the Spanish football league. I never sell shirts that are fake and, if I did, it certainly wouldn't be a £5 promotional t-shirt!
The feedback I received for the transaction was positive but the comment sated 'Not sure if it's legit though.' This may not seem like a big deal to some people but when you make your livelihood through eBay then your reputation is even more important than ever. A comment like that could put off potential buyers and prevent a customer from obtaining a shirt they want.
We all know the problem of fake shirts on eBay and we can all help combat it by reporting to eBay those selling them and refusing to buy them. Always, though, make sure before accusing someone they are fake. Check with the manufacturer and also think whether it is an item that somebody would fake. I would implore you, though, to never leave feedback doubting an items authenticity unless you are 100% sure as it can be very harmful to a sellers reputation, all because you may not be sure about an item but not taking the trouble to find out.
Another issue to be aware of is 2nd tier shirts. They are sometimes made by Nike and they are like prototypes. They will make a new design shirt for a start of a season and try it out for some of the pre-season games. The design may change though for the start of the season and so the 2nd tier shirts, although genuine, are different from the final shirt. An example of this is my purple Atletico Madrid away shirt which is slightly different from the final shirt. This does not make it a fake and I would ask, if any doubt, for you to check with the manufacturer before asking about the authenticity of Nike shirt. Their shirts should have the coloured size label with a code on that you can use to check the shirt. If it's not in the original bag then the code should also be on the swing label. Please always check before accusing a seller with a good reputation.
If we all pull together on matters like this then we can all be confident in who we are dealing with and play our part in helping to dry up the market for cheap fakes.