I had the same question. If I had to guess, I’d say they buy bundles of games often, where the bundle includes a game they already have but 4 games they also want. The price is right and the condition is good so why not? If a cart ever fails you have a backup right there. If the value goes up (likely) you can sell one of the copies down the line.
Yeah it’s flea market strategy. If you try to buy one game, non-knowledgeable flea market seller thinks that game must be valuable, jacks the price up, puts it on display. But if you act like it’s just a box of old games, haggle them down on it like you just want some old games for your old system.