If you sell private goods such as clothes and furniture that you bought for personal use, you do not have to pay tax on the amount. The same applies if you exchange your private goods with others.
If, however, you sell goods that you bought with the intent on selling them on, or you rent out private goods, you have to pay tax on the profit you make.
This applies no matter how the selling or exchanging was facilitates, including flea markets and Den Blå Avis.
It is irrelevant whether the goods sold are new or used, where you bought them or how long you have had these goods. The only thing that is relevant is whether you bought the goods to sell them on.
Danish tax legislation does not define what you are allowed to sell. However, other legislation prohibits that you, for example, sell stolen goods or euphoriants and that require a licence or occasional permission to sell alcohol.
This is what you do
If the sale or exchange is private: You enter your profit in box 20 in your tax assessment notice and field 250 in your preliminary income assessment.
If the sale or exchange is for business purposes: You enter your profit in box 11 in your tax assessment notice and in field 221 in your preliminary income assessment.