I am not an expert of the subject so I can't answer very well but a lot of them, for other items like TVs - they get items that maybe fell off the truck and are refurbished or whatever, or they get a shipment of crap they don't want and so they try to get rid of it, or various other reasons. I don't question it, I just revel in affordability.
Maybe they're trying to undercut Amazon. If the product is available brand new directly from Amazon, why would anyone choose a less reliable third person vendor charging more? I like to support small businesses, but I'm just looking at reality. It's my experience that third party vendors can only get away with really high prices when the item is no longer in print/production or is hard to find.
I've always assumed, if it's a new product on the market, people buy them, possibly in bulk, to sell them on, whether from the original distributor or another retailer/other party. But if they sell them for less than they paid, they aren't necessariily profitting, especially if the earnings are split with Amazon, so this doesn't seem like routine retail--I see what you mean. All in all, it seems like Amazon Marketplace can't be the sole source of income for most of the merchants who use it.