Despite the common perception that sales people have to be outgoing and extroverted in order to perform their jobs effectively this does not hold true for many sales positions. While it is definitely true that the vast majority of sales positions require extroverts and all things being equal an extrovert is usually the more suitable person to hire, in our experience it can be a big mistake to assume that just because an applicant is introverted he is not suitable.
Be careful though, there are introverts that can be very effective and there are introverts that will be complete failures. The key seems to lie in a combination of the other characteristics that they possess. Another important issue is the specific sales role for which he/she is being considered.