There are not to many fakes posted on ebay but when someone posts a fake, here are some things to look out for...
1. the name is incorrect, missing letters or takes of the little circle next to the name. For example 'Dark Big Rabbi' which is a fake name for Exodia
2. the card is blurry or faded.
3. the card is holo(shiny) when it is usually normal. cards are usually not completely shiny with only the picture and the name in shiny. most fakes are completly shiny, it is almost impossible to see the card picture with all that shine and the stars shine in a circle.
4. there is no shiny square in the bottom right corner or it does not display the millenium eye or the word Yu-Gi-Oh! when moved around in the sun.
5. the atk and def in the bottom right corner just above the shiny square is longer then a real card.
6. the word magic, trap or spell is a lot larger.
7 the stars are a lot different with real cards the right hand side of a star seems to be in a shadow as if a light is coming from the left hand side of the card, they also dont touch the edges of the circle the stars are in. with fakes they take up the whole circle with the tips touching the end.
well thats about all that is to know or find a fake!