Is it obligatory for someone who bought a phone at a price lower than the market value and gained a profit to return the amount in their possession to the owner of the money after the seller committed suicide and the money remained with the buyer?

It is not clear in what capacity the amount was taken. If you are a seller, you must deliver the device or refund the full amount to the buyer, and you can claim it from the estate of the deceased merchant. If you are an intermediary, you are not entitled to the amount (300 riyals) except upon delivery of the device. You are obligated to strive to deliver the device or refund its value to the buyer. If that is not possible, then you are not bound by anything, because a broker is only liable in cases of transgression or negligence.