The extra money is usually not a scam in itself if they are trying to actually get you to ship the item. Once upon a time scams were all about moving cash, but I surmise those have gotten harder, so now they secure your item and (presumably) resell it.

Thus, the promised extra money is actually just a way to both ensure you'll "sell" to them as they made the highest bid, and also perhaps an attempt at triggering just a touch greed which does wonders for putting the blinders on.