Networking. Getting to be good friends or associates with people already trusted in the business. Then they can vouche for you. Selling to people who already know you (such as the folks on this forum....I'd buy a het from you!) and then they can also pass along good reports when the hets prove out as advertised.
And of course, there are always people willing to give someone they don't know a chance. Always treat them right as well, and they will also pass the word along.
Building the sort of reputation that allows strangers to buy hets without question takes years of hard work and dedication to the industry and to your customers.
(That's my nickle's worth, anyhow.)