Well, I can’t really give you a one size fits all example but, the market price of an item is based on supply and demand, plus in mmos you need to also take into consideration a possible “bottom price” which is the same as selling the item to NPC.
Buying a resource from NPC with a fixed price isn’t necessarily the bottom price if the item drops from monsters - in which case the supply might be so much greater than the demand that the items true sell value is somewhere between selling it to NPC and buying from NPC (if they don’t equal).
On the question of controlling the market with large sums of cash:
Yes you might be able to raise or lower a price of a certain item or resource in market by buying a lot of the same item (= prices start going up since demand is higher than normal) after which theres a possible time frame in which you might be able to gather profit. However, on resources that are very common the market price will quickly become stable again since controlling a market with a very high supply is pretty much impossible.
The rare resources and items: In this case controlling the market is easier, since theres less of the item in sale in general. But even on this case, the resource you would need to buy yourself in large quantaties to make the price go up is wired to whatever you can craft from it. So naturally a certain resource can’t exceed a certain price. In the rare resource scenario you might momentarily get the price up (cause you are buying it - higher demand) but you might fail to control the market if the demand for the resource isnt high enough. If the demand is higher naturally than the supply, then the price is going up even without you buying large amounts of it. Ofc in that case the price will go up faster until it reaches its maxium price. One thing to understand is that market control isn’t easy or directly profitable. Like in the real world, trying to control a certain market has high risk potential.
Also, in order to control a market you need to be really aware of the currents in the market. Heres an example:
Back in RO1 euRO there was a few weeks in which there was a huge increase in bots that were farming Elu. Elu was a resource needed to upgrade any type of armour, so naturally all players needed it. Since Elu supply was suddenly very high due to the unnatural effect the bots had the price dropped from 50k to 35-36k. At that moment I bought Elu with all the money I had hoping that the bots are banned soon and the market will get back to normal. I waited for 1½-2 weeks and the price was back to 48-50k. At that time I sold the Elu for profit.
Wheres the risk in that? Well, the risk i took there was 1) the bots keep farming - elu price drops even more and all my capital is stuck on a resource thats now cheaper than when i bought it and will possibly take a very long time for me to even break even from my investment. 2) During the time my money is stuck on a certain resource, I can’t get any good deals I might see. (= easy profit buy - resells).
Playing the market is a skill which can be learned by practice or by reading books. I would suggest the latter.
I hope this helped a bit, even tho I realise my examples are only the tip of an iceberg when discussing this topic.
IMC pls!