Should I be concerned about a possible counterfeit DS game?

Solution 1:

As a seller of used games:

The games typically function as normal. Longevity is the concern. They're more prone to damage, data loss, etc.

If it's a counterfeit and you are US based, don't panic. In the US it is illegal to mail counterfeit goods. Just tell the seller "Unfortunately it looks like the game you delivered is a counterfeit due to X, Y, and Z (don't blame or accuse them). I'd like a refund." If/when they ask you to return the game inform them "Sadly, It is illegal to mail counterfeit goods in the US. I still require a refund". Escalate as needed. You'll get a refund and still have the game to tinker with. I speak from experience :P