The company does not allow employees to sell vacation days, but they can carry a limited number of unused days over into the following years.
Brand said the program is open to about half of the company's 15,000 U.S. employees, so as not to interfere with production. About 2,000 workers used it last year.
In most plans, the employees aren't actually buying and selling time from each other, Stich said. Time sold back to the company goes into a pool and in certain cases, other employees can buy it. New employees who get limited vacation time during their first few years at a company are among those who appreciate the benefit most, she said.
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Sandi Winant, a 24-year employee at financial services company USAA in Colorado Springs, said she buys a week of extra vacation time every year to work on a side business selling salsas and dips at festivals and events around country. She plans to pursue the business when she retires in a year or two.
"It's spread out over a whole year's paycheck so you don't even notice it," she said. "This gives me that extra padding to do what I want."
Jeff Weiss, senior vice president of benefits at San Antonio-based USAA, said about 41 percent of the company's 25,000 employees bought vacation time last year, while 11 percent sold it.
"We think time off is actually critical to productivity," Weiss said. "When people take their time off to refresh and renew, we believe they service the members more effectively."
Some employers have set up flexible benefit plans where employees can sell back their extra vacation in exchange for a different type of benefit they find more valuable, such as more disability insurance or life insurance, Stich said. An employee can get credits they use to buy benefits and then trade in the dollar or credit equivalency of those vacation days in exchange for other benefits they want.
Evren Esen, manager of the Society for Human Resource Management's survey research center, said more companies are moving toward PTO plans that give employees more flexibility in determining how they want to use their leave.
"In terms of human resources it's easier to manage it," Esen said. "They put everything together in one bank and don't have to separately track sick, vacation or personal days."
The PTO plans also encourage employees to schedule most of their leave time, which can be more reliable for the company than workers suddenly taking a day off on short notice, Esen said.
The number of companies with PTO plans has grown from 42 percent in 2009 to 52 percent this year, according to the group's annual survey of more than 500 randomly selected human resource professionals.