The calendar – which is made by toy firm Creative Kids – usually costs £7 but has since sold-out in shops.
Cheeky eBay sellers have been keen to cash in on its popularity of the calendar by reselling it for up to £40.
Home Bargains told The Sun that they won't be getting any more stock of the calendar this year and Amazon is also sold out.
Behind each of the doors there are 16 different pots of slime, four pots of glitter and two pots of sand.
The gunge has become a playground favourite and while it can be made yourself at home, you can also buy pots of it from toy shops.
Savvy shoppers can also have a go at making their own version of the calendar with a DIY kit for £18 from eBay.
If you're not keen on the idea of more slime getting into the carpets then maybe theLego advent calendar is one for you – and here's a hack that let's you get one for free.
Reese's has launched its Christmas range for 2018 and this year it's offering peanut butter fans two advent calendars to choose from.
Other retailers have also been announcing their festive ranges for 2018, including Aldi who is bringing back its wine advent calendar from November 4.
John Lewis has also opened the doors to its Christmas shop, where chocolate fans can create their own pick & mix Quality Street tins.
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