If the seller is a private person, you should check the platform’s instructions to see whether the platform can provide assistance in finding a solution or mediation.  If the complaint or mediation does not result in a solution, the buyer can sue the seller in the district court.
If the sales or other agreement is between an undertaking and a consumer, the buyer can assert their right to consumer protection and ask for help from Consumer Advisory Services. In other instances, for example, when problems arise between two private persons. the consumer authorities have no jurisdiction.
The selection of a payment type may be decisive if problems arise. Read more on the topic on the Finnish Competition and Consumer Authority website.