Does the merchandise leaving the store in a transport vehicle to the neighborhood count as possession and قبض (qabd, taking possession) that permits its sale to the one ordering it for purchase in installments, or must it first be unloaded at my house?

Possession is achieved by anything that is considered possession in common practice, and reference is made to people's common practice in this regard. So, if one buys a refrigerator, its possession occurs by taking possession and moving it from the seller's warehouse, and it is not a condition that you take it to your house. As soon as that occurs, you may sell it to whomever you wish.