At the time basically, in that town, that particular kind of salesperson had few similar other places to go for a new job. Because they were newly married, or had kids, or bought a house or a car, then it meant they had increased financial obligations necessitating they really needed to keep a steady job and get paid well. Meaning that person would be less likely to suddenly quit if they were unfairly treated by their employer.

He was bragging about how he liked when he had a salesperson that worked for him in that situation so he could take advantage of it. It's a potentially more old school term he'd say, he "had them by the balls" or basically had the upper hand to be a shithead.