Yeah, there is definitely a cycle. Generally speaking players fluctuate by up to several hundred coins throughout the day based on demand from the euros, and this is the easiest way to flip players On top of that, you might find someone who is doing what you were doing yesterday (flipping the card) that will drive up price for an hour or two while they are fixated on that player.
Generally speaking, I find player prices are highest while Euros are playing a ton of games (and demand for players is at its highest). On weekdays That's generally between 2pm-8pm Engerland time (9am ET - 3pm ET). On the weekends you obviously extend those times earlier in the day as the kiddies don't have school. The exception to this is if instead of playing a lot of games, the euros are opening a lot of packs. That happens on Wednesdays when the TOTW is released or during any of the "happy hour" specials which are usually on Fridays. When Euros are opening packs instead of playing, prices drop as there is a huge influx of supply.
Anyway, long story short. If you are looking for the cheapest time to buy non-rare players, it is usually in the late afternoon and evening US time while all the euros are sleeping (If you are talking about an IF player or really rare player this might not hold because of supply shortages).
That daily fluctuation in price is the key strategy to the 300/350 game and the 600/650 game that I've used for years. Buy players in the late afternoon and evening US time when their value is down and list them overnight for 12 hours (+6 hours in the morning if they don't sell on the first listing). You should be able to easily make 25-50% profit on a full trade pile (don't forget the 5% EA sales tax).
