If you want to sell something yourself, you have to register in advance for the flea market and rent a stall there. To do this, you usually have to contact the person who organises the flea market a few weeks in advance. You can find the contact details of the organiser in the flea market ad. Read more in the section "Where can I find a flea market?".
When registering, you have to state how much space you need and clarify whether you need to bring your own table (or the organiser provides it for you). Flea market places often have different prices. The stalls at the entrance and exit are usually cheaper- because people buy from the stands located in the centre of the market more frequently.