Sierra Poulson at her home in Omaha, Neb. Ms. Poulson, who has donated her eggs three times, helped start an online forum for egg donors.Credit...Ryan Henriksen for The New York Times On their websites, next to glossy pictures of babies, some fertility clinics and egg-donor agencies refer to eggs as a “priceless gift” from caring young women who want to help people with fertility problems. There i