Here's a dirty secret. I screenshot the market, then every time I notice any number has changed, I compare it vs the screenshot.
(...Hey, don't give me that dirty look, this thread -is- about OCD)
You can tell if it's the algorithm vs other players by the yellow numbers. Bottom left is the amount of commodity owned total by other players (not including yourself) and bottom right is the average price paid, which is usually a stupid and useless piece of information because it's skewed by people who bought at the start of the game then never ever log in again.
In my experience the fluctuation in direction is over the course of maybe 1-6 hours, but the idea is that the closer you catch the system in right after a change in direction, the more profit you make, hence the more frequently you can check, the better. I personally have the window open in the background while working on assignments/projects, and printscreen-ctrlV takes 5 seconds every 5 minutes.
Luxuries generally are a lot less lucrative than capitalizing on resource spikes (20 units bought at 200 and sold at 350 = only 3000 profit early-mid-game for example) but the more people you chase away through the course of the game, the better you set yourself up to rake in serious cash when it goes between 600 and 1200. I also updated my assessments in the start:
Culture: Dependant on position in game; it's a factor of the price of great persons and what your own current culture-cap-for-GP is. Can go between 100-200.
Luxuries: Updated to 65% target from 50%. Made a lot more sense as you rarely ever see prices dip below 50% of max, especially early game.