"I contacted the seller and he replied me by saying FOV on 2.8mm is the same as 3.6mm on 720p camera." - that is a blatant lie. Focal length and field-of-view will depend on sensor size, not resolution. Other numbers don't mean anything.
is your shot wide enough? If not, get the seller to fix their mistake, and pick up a 2.8mm lens camera. It's as simple as that. Nayr is playing with numbers to show that because you get a wider shot at 2.8mm, you also lose detail from subjects further away from the camera - they will be on fewer pixels. Higher resolution simply lets you get more detail in the scene, potentially allowing you to go wider on the focal length.
Picking up detail is all about filling the frame with the subject. You choose your focal length for that. In general, 2.8mm works great for watching an area, but 6mm+ (up to 100mm or higher) is useful for catching details (license plates, faces, etc) of a subject when they pass through that area. Backyard gates and doors are areas where longer focal lengths (more zoom) can be beneficial. As you can imagine, it can be good to have one longer-focal-length camera for every wide angle camera you have. Gather a lot of views with more detail (6-12mm can work well), and also pick up the general area with wide-angle to see everything that occurs. That or install 20x 6mm cameras on just the front of a house - catch all the detail, one narrow field-of-view at a time.