I haven't bought a game on e-bay for a couple of years now, but I happened to be looking for some reason tonight, and as I progressed, I began to realise what its biggest problem is:
How do you know if it is real or fake? 
 
This one as an example of honesty is interesting: 
  This is my sons used Yu-Gi-Oh World Championship 2008 Nintendo DS game.

He recently upgraded his DS to a DSi and the game does not work on the DSi (indicating that it is a copy not an original), we have tested it and it still works on a DS.

For a used game it is in well used condition with marks and creases on the instruction book and the cover has tears on the plastic, marks on the cover on the bottom.  If you want send me your emails address and I can send you a larger picture.

I'm way too suspicious to buy anything on eBay let alone online. I know it doesn't happen often but I really, really hate frauds and cheaters. I actually remember one case last Christmas when a mother wanted to buy a 360 for her son on eBay. Found a good deal and paid for it. A few weeks later they recieve an A4 envelope with a picture of a 360 in it.
 
So be careful around this Christmas when you're buying anything online folks!

" I'm way to suspicious to buy anything on eBay. I know it doesn't happen often but I really, really hate frauds and cheaters. I actually remember once case last Christams when a mother wanted to buy a 360 for her son on eBay. Found a good deal and paid for it. A few weeks later they recieve an A4 envelope with a picture of a 360 in it.  So be careful around this Christmas when you're buying anything online folks! "

I buy a fair few retro game on the site, but I have yet to buy a DS game. The games I buy there have little chance of being fake... I use all my powers of deduction, everyone needs to when buying stuff online, even with reputable sellers. 
 
Fools rush in.

" I buy a fair few retro game on the site, but I have yet to buy a DS game. The games I buy there have little chance of being fake... I use all my powers of deduction, everyone needs to when buying stuff online, even with reputable sellers.  Fools rush in. "

Make sure the seller has good feedback and well isn't located in China/Hong Kong/etc, I know i've purchased a few games from Hong Kong cause they were cheaper and am almost positive after the fact they are fakes but they play fine on my DSlite.
 
I've never had issues with eBay personally besides damaged or misrepresented items and ALWAYS got a refund.

" Make sure the seller has good feedback and well isn't located in China/Hong Kong/etc, I know i've purchased a few games from Hong Kong cause they were cheaper and am almost positive after the fact they are fakes but they play fine on my DSlite.  I've never had issues with eBay personally besides damaged or misrepresented items and ALWAYS got a refund. "