When an item is buyable, supply increases and value drifts down. When it disappears from the shop for a long stretch, supply dries up and value climbs. That is most of what value movement is.
The buy window
The cheapest moment to acquire an item is while it is in stock, because you can pay the shop price instead of the trading price. Compare the two before deciding to trade for something.
The trade window
The best moment to trade one away is after it has been out of stock for a while and the value has recovered. Watch the change log rather than guessing.