Why do people want to buy full sets of colors and small cheap items? If they aren't in high demand the market would be slow and nobody would sell or buy these cheap items or seemingly useless sets when so many people just have custom textures when they want a cool looking tori, but that is not the case since so many people are buying and selling the most random colors and people always offer on them. What is the source of all the demand on every color? Is it simply to make profit or is there a reason that people actually want all of these?

Well people want full sets most likely because the achievements. That's why I get them at least. As for why people buy misc items it's either to 1. Make a full set of the color to get the achievement, or 2. They're being sold for really cheap thus able to possibly get easy profit.certain misc items are usually cheaper than getting textures for whatever they want.

I personally don't buy single miscellaneous items because they aren't really that easy to sell and there isn't really an astounding demand for them

Like Leaf said mostly for the achievements. Also for marketing purposes though. If they see a chance for profit they take it regardless what the item is. Another thing is people prefer actual color items rather than textures that look like a certain color because of the value of it It's the same reason you don't buy a knock-off product

Do achievements give anything special? If not then what is the point in buying a full set for the achievement?

Achievements does not give you anything special, it just looks good. And most of marketeers do marketing for getting TC