Regardless if there is ethanol on the boat or not, the previous owner may have used ethanol fuel. If the fuel lines are not resistant to ethanol they could be damaged and preventing fuel from getting to the motor. I had to replace a fuel line in mine due to ethanol damage, no fuel was getting to the motor.
Using that method, how long did it take for the bulb to expand? Once it was fully expanded and holding it vertical, did any air come out when you squeezed it? Did you get a nice stream of fuel when squeezed? Did it refill itself again?
Side note: When I install the fuel/water separator, I'm guessing I need to fill that up all the way with fuel before assembling it into the mount, as I'm reading in the past pages?
More on the ethanol subject Fernando brought up. I have seen fuel lines that were soft a gummy from ethanol. I mean REALLY soft! The line could be collapsing under suction. Something else is going on here. Your finger trick is basically a check valve. The same thing the check valves in the primer bulb should be doing. Strange... I'd like to see that thing in person.