At flea markets individuals may sell their goods themselves or leave them to be sold by a flea market operator.
The Consumer Protection Act covers transactions between consumers and businesses. It does not, however, apply to transactions between two private individuals. The transactions you enter into as a seller at a flea market therefore are beyond the scope of the Consumer Protection Act and the consumer authorities have no jurisdiction to handle any disputes that might arise from such transactions.
The contract between you and the flea market operator is governed by the Consumer Protection Act. This means that the marketing and contractual terms of the flea market can be assessed within that legislative framework.
The flea market operator is responsible for ensuring that goods left to be sold at the flea market are under appropriate supervision. The flea market operator is generally liable to compensate you for losses arising from e.g. theft unless they can prove that the losses were not a result of their neglect. You may claim the previously agreed sales price as compensation for lost items.
The flea market operator may be liable to compensate even if their terms of contract stipulate that lost items are not compensated for. If you are unable to settle a dispute with a flea market operator, contact your local consumer advisor.