I've done it a few times for ds games that I have that are cart only, I assume it's just a print out from a computer but I can't tell a difference. If I ever trade or sell the games will the buyer know? Is there some way to tell?
Yes, I would definitely know as well. I make my own replacement cases using various sizes and the fact that it's extremely difficult to replicate boxes (although I've seen some AMAZING tutorials on making aftermarket boxes through lamination techniques), makes it seem like it would be apparent to anyone who didn't know better or what the original boxes looked like. As @Forgotten_Freshness said, just make sure the "replacement case " or "replacement case art/insert" is listed on your items and there should be no foul play suspected. A few members I've traded with have been quite happy with the replacement cases and custom artwork, but I still only traded them as "disc/cart only" in value unless the trader was exceptionally enthusiastic about receiving custom/replacement artwork.
The games I've got replacement case art for are solatorobo, little red riding zombie bbq, and dokapon journey. the paper certainly seems like actual ds case paper. I guess the white side of the paper isn't as shiny/smooth. The case itself is just a case from another game.
OK, i just got some new ones in today, they are pretty legit. The seller is bosslevelgames they look and feel like the original case covers. I am astounded. I bought another one, for little red riding hood a while back(from another seller) and it is clearly not as good of quality.