I bought a Diamond, Pearl, and Plat 3-pack from San Diego, Cali on Ebay for 34.99. Once they arrived, I immediately checked them for givaways. Each checked out both with their icons, getting read by dream radar, and their cartridge design, and they all play well.
However, they had a harder time fitting into the slot on my new 3ds (my other carts did too, but these were more noticeable, possibly due to their newness) and the stamped text on the back was a slightly different size than on another cartridge of the same game at Gamestop.
Have I been cheated?