Sell in the spike
First of all, you can note that for almost every item on the Steam marketplace, there is a huge initial spike in pricing that takes some time to correct. I don’t know whether it is due to Elijah Wood making it rain to have the newest shiny items, or because some OCD basement-dweller has to collect them I don’t know. Surely there is an element of confusion in the market before rational pricing can be established as well – but whatever the reason, the spike always seems to exist. Given this fact rule number one is: Sell during the spike if at all possible. A great example of putting this into practice is to sell the first trading card you earn in a Steam Sale immediately.
Pro Tip If you are prepared for the sale, you’ve likely set aside some other sets of non-Sale Trading Cards for badge-craftage. Plan to cash in all or most of these on the first day of the Sale to get early Sale Trading Cards that you can sell while the price is high.