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.
Assistance with migration to Serbia. Residence permits and permanent residence, opening of sole proprietorships and LLCs, assistance in finding housing, choosing a school, tutoring Serbian, translation services. Free consultations. Payment after work is done.