"sell first" works if you have such a unique solution/value such that for the customer it's a "either this demo or nothing" situation. that's exceedingly rare.
most of the time there's a healthy large existing set of solutions serving the same market so it's difficult to justify using something new.
even if you have some unique selling points they may not be strong enough or they may be canceled out by the lack of other features and maturity of your new product vs existing established ones.
for example let's say your product is a customer support software. how do you "sell first" when there are so many mature existing companies solving the same problem?
how long will it take you to reach the same capabilities, support, security, reliability of existing solutions?
are your differentiating points really strong enough that someone's going to say "screw Zendesk I'm going to try this random new one-man product because they have this feature X"?