Now if you find a house that you really like then it costs nothing to leave a card through the letter box. Still, I guess homeowners might be a bit suspicious and not comfortable going alone without estate agent (if they'd consider selling at all)
a) you can do it entirely from your computer
b) you don't have to drive anywhere or go door-to-door
c) you get feedback on who opened and read your letter
d) you're guaranteed it'll go to the homeowner, not just the tenant (if it's being rented out at the time)
e) you get a professional introduction that signals to the homeowner that you're serious; more so than someone that just prints off 50 copies of a letter on their home printer and drops them like flyers on doorsteps.
In addition, unless the homeowners have also previously signed up with your service (in which case why not an estate agent?) then, IMHO that's still a flyer on the doorstep (as opposed to manuscript and personal letter if I leave a card).
A professional introduction by whom?
A web only start up that "introduces" two perfect strangers for a fee?