There are so many fakes on ebay that it seems impossible to distinguish between a genuine and a clone.
However heres some tips to remember:
1) Always see if you can get close-up pictures. See stiching quality, fakes are poor.
2) Buy trainers with boxes and look for 'non-damaged' ones. Dont buy from Europe, states who don't sell without them. Boxes should be correctly labeled and not hidden in the advert. This is a huge flaw!
3) See the heel of the trainers, many fakes have faults with exact production of symbols. New shox have the 's' cut into the rubber.
4) Buy from seller who guarantees 100% genuine or money back.
5) Shox are widely copied so if your bidding on a 'BNIB' pair which retail at £120 and your paying £30, 99% likely that its a fake. Real shox are much lighter than the fakes. Fakes are heavy like a bricks on your feet.
6) Check negatives of seller to see any Fake or item decription problems.
7) Check well known branded retailer websites to see if they sell the same model! Alot of clones are not even on general high street retailers shelves!
8) Do some research on forums like instyleshoes.com and see what other people are saying about ebay/internet retailers
9) And finally use your common sense! Nikes are not the only
sellers of good shoes, try adidas or reebok too.