If you go to the steam marketplace, and you apply the normal filter, you immediately see that the first items are stock items, or the items that you start with. How does a player get those items in trade-able form?
According to the wiki page on the Normal quality, there exist a minority of items with the Normal quality that aren't supposed to be, and/or have the kill-counting abilities of Strange weapons. It seems that that's what you're seeing on the market.
Those "stock" items are the result of players applying Name Tags or Description Tags to stock items.
When that is done so, instead of modifying the stock item, a replica of the stock is created with the properties applied to it "changed name, changed description".