To make a Skull Candle it's very simple. You just need a skull You can find a skull in some parts of Tibia as decoration or you can get it as a loot from an Amazon for example. You also need a candlestick To get a candlestick you can buy from many equipment shopkeepers. Click on the links to get more information if need be.
Click on skull and use it on the unlit candlestick to make a Skull Candle You can turn on or off the skull. The Skull Candle will only remain lit for 50 minutes in your house or backpack. After this it will turn back into a skull in which you will have to make it into a Skull Candle again. The timer doesn't reset if you turn off and on your skull. You can keep it in your house unlit if you want and it won't disappear.
To sell the Skull Candle it needs to remain unlit and unused, otherwise it won't appear in the market. This means you can't sell a Skull Candle that you make as it automatically is lit when you make it.
You can also get a unlit Skull Candle dropped by Pirate Corsairs which is considered a very rare drop (less than 0.5%)
Lastly according to Tibia Fandom:
[The Skull Candle] can also be found at Daily Respawns in Dark Cathedral and Kraknaknork's room in the Rookgaard Orc Fort. Can also be seen on the Isle of Evil and in the Magician Quarter/Residential Area.
A good question to refer to regarding other expirable items would be https://www.tibiaqa.com/12977/what-are-the-illumination-items-that-expire-and-long-does-take-them-expire?show=12977#q12977