You got a few factors here, lets take the example of warp scramblers here.
In order to have a supply you need people that do manufacturing and people that find them during mission runs, the way these people sell will vary, but the manufacturer will most likely set up a sell order on all his products to gain profit out of it by using materials on it. Warp Scramblers give a nice profit by buying mats and making them and setting up sell orders, however, they loose if they sell to buyers because of how low it is. The same applies to T2 variants.
Rarely there are products a manufacturer can make that will give a decent profit by selling them to buyers, because as you might know by now, the prices buyers ask are low, and if the manufacturer in question does what I do, buy minerals to produce and sell products, he is most likely to set up sell orders. That is one big source of products you got out.
The other source is the people that find them during missions or pvp and just feel like dumping it to people willing to buy it, simple as that.
How well it sells it depends on the item itself, because the majority of the items manufactured do not give a good isk per hour when making it with mats and selling it to buyers, hell in most cases the manufacturers are better off just selling the minerals rather than spending them on something that will give a negative drop on their wallet.
Not to mention that people willing to play the 0.01 isk game are going to have a much higher chance of grabbing an item before others do, and that includes to people trying to sell the item through sell orders.
If you see people with thousands of an item they either make it themselves or they buy the items through buy orders because it is a very well traded item like minerals and other famous stuff