I see this all the time in MercadoLibre, the most used e-commerce platform in my country (and all of Latin America I think). If you want to buy something you should hit the "Buy" button, which puts you in contact with the seller and also charges the seller a small fee. But you also have a small Q&A section where you can communicate with the seller; and in there you will see many clever attempts of communicating some contact information (usually the phone number) between the parts to avoid paying that fee. The section is automatically (and I think manually too) moderated, but it looks like a never-ending war where the platform is always losing.
