I don't approve of this. Your basically exploiting Nintendo and us preowned buyers have no chance of a free game.
Hate to break it to you but Nintendo gets $0 out of the pre-owned purchase so Nintendo is definitely not intending for a free game to go to someone purchasing pre-owned. The free game deal also only runs until July 31st so unless you got a pre-owned copy in this first two months (which you're most likely paying $50 - $55 for) you're not getting the free game anyways.
*Here is a copy & paste job that I previously used to explain why this isn't as evil as it initially seems.
I already own physical copies of every one of those games, so Nintendo didn't lose sales on the digital copies...I was never going to purchase them digitally anyways. Nintendo actually just sold 3 more Mario Kart 8's at a retailer from me doing this which Nintendo already assumes the free digital game is going with each copy. The sales numbers increase for Mario Kart 8, even if they're padded from that promotion it helps Nintendo to some degree. I don't know what outweighs what, the fact I'm buying digital copies for free that I already owned physically or Nintendo has 3 more physical sales of MK8, I think the MK8 sales should be more significant. Obviously everyone who takes advantage of this won't have all the physical copies like me, but again Nintendo should account for one copy of Mario Kart having one of those digital games with it.
Best Buy is selling games and taking the trade in at the prices they have determined. That is their decision. They're not going to have any difficulty selling used copies of Mario Kart 8, with the history of past Mario Kart games they have held their used value higher than most other games. This shouldn't be a title that will be in the bargain bin used, it will be highly sought after for years...especially if Nintendo releases DLC for it.