Teresa Estes, a licensed mental-health counselor, watched as business at her private practice dwindled last year. Then the single mother turned to her keyboard to boost her income.

Ms. Estes applied to become an "expert" on LivePerson Inc., a Web site where clients pay for online chat time with professionals and advisers of all stripes. For $1.89 a minute -- a rate she set -- the 39-year-old from Marianna, Fla., dispenses advice to clients around the globe. She spends about four hours a day logged on to the service, often...