When a new item is released, it almost always starts out with a high price. This is because nobody truly knows what anything is worth until they start selling, and at first they are rare as most people haven’t had a chance to get one yet. There has been one outlier to this, which was Chronoscrolls in the past, but as far as I know there haven’t been any others so far.
What you want to do at this stage is to sell your item(s) in preparance for the future.
Post-Release
Once the item has gone through its first few days (sometimes just hours), the price will start dropping. This is part due to supply and demand, and part due to the balancing of figuring out what its real value is. As time goes on, more and more players will be obtaining theirs, and the S&D part takes a bigger toll, essentially dropping the price pretty low.
Final Stretch
This part isn’t always the same, and it depends greatly on the items. Basically this is based on desirability. Although there may not be a lot of supply after an event ends or a sale stops, there may also not be any demand. As a result, the price would stay low. On the other hand, if an item is very desirable, its price will start rising and over time will become more valuable.