A contractor may employ many subcontractors to create a project. Builders are often subcontractors who do the actual installation. Some large contractors have their own plumbers or builders.
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.