What's the deal with the single-type-of-piece bags on Amazon? While looking for current Lego offers on Amazon, I was surprised to see loads of offers such as:

10x Lego inverted slope 2x2 dark red
8x Lego plate 4x6 new dark grey
40x Lego Technic pin long with stop black
40x Lego plate 1x2 light gray
25x Lego brick 2x2 new brown

and so on.
It seems quite some of those parts come in ... peculiar colours, such as lime green or lots of browns, but I may be mistaken and this might change day by day.
What are those offers?

Is that vendors who buy loads of original Lego sets, put the individual parts into larger bags of 5 to 50 pieces, and then sell them and somehow have a little revenue margin?
Is that vendors that somehow get large quantities of the same pieces from Lego (e.g. through channels that allow toy stores to build up some custom scenery around the actual sets in showcases if there is such a thing) and then resell packs of such pieces?
Or are those offers not for Lego pieces at all, but for some rip-off that fakes the brand logo on the studs?
I think it's really hard to tell for sure without actually making a couple of orders and seeing what arrives. What I can say is that it appears that they are selling at a HUGE markup over similar parts available on Bricklink. (A quick check shows an average markup of 600% over BL on several lots)
I would guess that either the first, third, or some combination of those two guesses is likely to be the case here. Since secondary markets like BrickLink and BrickOwl are somewhat niche sites maybe the seller(s) are taking advantage of people unaware of them and selling at a very good profit to a larger market. Additionally since Ali Baba, Ebay, and Amazon tend to have a large number of counterfeit sets available these sellers may be able to get away with selling all or a mixture of counterfeit parts to increase their margins.
As for your second guess, there are programs that The LEGO Group makes available to organizations of various sorts to acquire large numbers of pieces at fairly reasonable prices however they do tend to monitor the secondary market fairly closely for evidence the parts from those programs are being resold so, while certainly still possible, it is less likely the parts are being sourced that way.
Again though, it is nearly impossible to tell simply from the advertised lots without actually making a purchase to verify what you would get.