Find someone with real start-up sales experience with strong communication skills and is naturally inquisitive.
Real sales experience is someone who's sold a product or service from alpha stage through to full release. This person has shown the ability to develop trust with the customer base and can transition the user from alpha/beta tester to paying client (compared to someone that may have been an "account manager" at a large company).
Communication skills are vital when the technology blows up she needs to keep your customers calm, while also clearly defining product requirements back to you and the product team. The communication skills will serve you well for early marketing and blog content.
A naturally inquisitive person will explore the technology and want to know how your technology works under the hood. This is invaluable in your relationship with that person, and is hugely beneficial in the sales process with customers.