Stock Levels and Prices:
Trading Strategy:
Example with Fuel Trading:
Optimal Route Example:

Monitoring and Adaptation:
Production Chain Strategy:
Map Overview:
Thats the time to buy things. If you buy items while theres only a handful of them, you pay a premium for them.
Same goes for selling items, if the stock is high the price will be low. So sell items that are not/low in stock at the buyer.
Best gain will be had if the seller has 500 in stock and the buyer has zero.
The trick is to find/create that combination.
An easy start into trading can be fuel. It has one seller, the fuel depot, that restocks relatively fast, and several buyers, the fuel stations. Most of the time the fuel stations will have under 100 in stock, but check that first. If its completely empty, then you got the chance for the highest profit. Grab as many fuel as you can at the depot and deliver them to the fuel station. Theres a chance that someone else does the same thing, on yours or on another server of the same map, so you might end up delivering fuel to a station that suddenly has some in stock, as long as its not over 100 you will still make an ok profit.
While on the road to grab or deliver fuel, check other traders stock. If you see one that has zero stock of an item they buy, go take a look at the trader that sells that item.
Theres better and worse routes. Personally, i like to drive circles, or rather multi stop routes, to avoid driving around empty.
A good example is the wooden box on east coast. The carpenter making them is at the north-west map edge, the port buying them is at the south edge, close by is the sawmill selling planks that the carpenter wants. Inbetween is a straight road that allows for full throttle on pretty much anything for its entire length. All other things that the carpenter needs are relatively easy to get as well. Exception might be the carp, but that depends on the car. 4x4 recommended.
Try entire routes, see where they are on the map, try to find routes that loop back on themselves, and keep an eye on stock levels.
Some productions require several items, so you can check them and see if any stock is low for them, and refill them to start production of the output item, which in turn makes it cheaper for you to buy and makes more profit.
Many items will come from places which need fed items to create, for example anime figures need glass, which is bought from float glass, but if stock is running low there, you can make lots of money by taking sand there to create glass first, then transport this to make anime figures.
Take a look at the trade overviews in 🧐-info-and-guides or at the bigmap (press M ingame) if you want to know what goes where, and its location on the map, if you prefer to always have the map open you can use the online maps.