Some time back while looking for Nintendogs for the DS Lite on Ebay I came across a supplier in Asia who was selling these games much cheaper than in the UK. The seller had excellent feedback, had sold hundreds of games and everything looked fine.
I purchased the game saving about £10 overall and about 12 days later it arrived and looked fine. Shrink wrapped box and everything seemed in order. The game worked fine and I had no reason to believe it wasn't genuine - it certainly looked it.
After a few months however the game started to crash on occasions and while purchasing some other games in a UK store which specialised in the DS Lite I took in the DS and game to ask for some advice.
The supplier quickly advised me that the game was a "high quality fake". The cartridge was a slightly brown colour and the actual inlay card in the game's box had started to fade. Looking closely at the Nintendo logo of authenticity you could see it was slightly different to the same UK versions. I was told this was a professional fake which is common in countries where trading / copyright laws are less stringently enforced. I did try and contact the seller but no response so I wrote this off as a loss and bought a new UK version from the high street shop.
The point here is that these fakes are so close to the real thing they are very difficult to spot and as such the buyers of these items had given the seller excellent feedback (as I did at the time) having not realised they were fakes (and therefore illegal).
I am not for one minute suggesting that all NDS games from abroad are fakes but you need to give this some consideration when purchasing such items outside of the UK and don't go on feedback alone. If you do purchase such an item, I suggest you thoroughly check it when it arrives and compare the inlay card and game cartridge to other NDS games you have. If it looks suspicious straight away then you have some level of protection through Paypal and Ebay
As well as being illegal, fake games could potentially damage your console (as I was told) so it really is something to beware of