Before we start, I'll go over the base prices and what Market Fees are.

Every resource has a base price. This is the default price of every resource and is the price that resources will naturally go towards when the price has been adjusted. Also, you use Energy Credits when trading resources, so if you buy something, you spend Energy Credits, and vice versa when selling, you gain them.

These are the base prices for all of the tradable resources:
Basic Strategic Resources are Rare Crystals, Exotic Gas and Volatile Motes.
Advanced Strategic Resources are Living Metal, Zro and Dark Matter.
Note that Nanites and Minor Artifacts have no base price, since they aren't tradable on the Market. Nanites are not tradable at all, but Minor Artifacts can be traded between empires through Trade Offers.

When trading on the market, whether it be the Internal or Galactic one, you are subject to Market Fees. This fee is a percentage value, which by default is 30%. Note it cannot go below 5%, and the highest possible fee is 60%.

The fees can be influenced by the following:
It should be noted that Insider Trading and Adaptive Programming are mutually exclusive, since one is for normal empires and the other is for Machine Intelligences.

In Stellaris, there are 3 types of Markets. Internal and Galactic Markets both deal with resources, but behave a bit differently from each other. The Slave Market is for... well as it's titled, selling and buying slaves.

When you start a game of Stellaris, you will immediately be given access to the Internal Market. Here, you can trade Energy Credits for other resources.

You can only trade resources you have at some point acquired. This means you cannot get something like Exotic Gases before you produce at least one unit some other way. Only you can influence the prices here. These are the key differences between the Internal Market and the...

You can access the Galactic Market once it has been founded in the Galactic Community, and so long as you are a member. For it to be founded, the resolution regarding the market has to be passed, and the nominations to be finished.

You can trade any resource that at least one empire apart of the Galactic Community has ever produced at some point. That means you can buy resources you have never ever produced before so long as someone else has. Additionally, the prices are now shared between all empires apart the community, and the prices will stabilse much, much quicker.

Whoever holds the Galactic Market gets a -10% Market Fee, as previously mentioned. It will permanently remain there until a resolution to move it is passed, restarting the nominations. But it can also move if the system it is in is under occupation by a community member or genocidal empire or if the starbase is destroyed. If any of these happen, then it will be moved to a random community member's capital system.

The 3rd type of Market, the Slave Market is unique. It follows the same requirements as accessing the Galactic Market, since they are technically under the same Market, but here, you can buy and sell slaves.

Note that Market Fees do not apply here. Any empire that has forbidden slavery in their policies will have to pay double the price to buy slaves, and said slaves are then turned into free pops, and are unable to sell any. The empire that sold the slaves will recieve all of the credits that the buyer spent, including the double price fee.

This only applies to the Internal Market!

When automatically trading resources, or doing a one-time trade on the market, prices will go either up or down, depending on if you are selling or buying said resources. The price however, will slowly return to it's base price over time. As such, there is a "sweet spot" of how much you can trade per month, since the prices will return to normal each month.

Note that buying resources will inflate the prices more than if you sold the resources, as such, you can sell more than you can buy.
These are the maximum amount of resources you can both buy and sell per month: