Is there a 14 day guarantee or something where the seller is required to refund or replace faulty merchandise?

I bought a new phone on Allegro on Monday, today it suddenly stopped working. Sony online support confirmed it's a faulty device and said to contact the seller for a refund or exchange.

I contacted the seller today he said to go to a Sony Store to start the warranty process. I'm wondering however if the seller has a requirement by law to replace the faulty device if it's within some time period? I bought the phone 5 days ago. The seller is a GSM reseller and I got a Faktura VAT 23% with it.

If I remember correctly items bought from internet can be returned within 10 days without explanation

Even if it's damaged?

Yes even when it's damaged. But it depends if you damaged the products or if it already was damaged before you received it.