Never pay down a deposit in a bank account until you have met the seller, seen signed a purchase agreement.
No serious private advertisers ask for a down payment before you meet.
Receiving an email with an in-scanned ID does not mean that you have identified the sender. You do this on the spot, when you sign a purchase agreement.
Selling a new pink bicycle with documents, purchased in Novi Sad, serviced, used for a couple of months. Price 20,000 dinars. Bicycle lock and lights as a gift.
