[1][2] He is the owner, chief expert, and president of J. Greenstein & Company, an auction house in the United States dedicated to appraising and selling antique Judaica.

[8] At 14, he left yeshiva in Flatbush, Brooklyn, transferred to James Madison High School, started working at The Gold Bug, and began collecting Judaica.

[4][9][10] He visited flea markets and garage sales to add to his Judaica collection,[11] and eventually began dealing in antique Jewish ritual objects.

[20][21] J. Greenstein & Company has also hosted and sold celebrity Judaica including Sammy Davis Jr.'s personal menorah, Rabbi Shlomo Carlebach's piano, a Seder plate that belonged to Joan Rivers, and items from victims of the Bernard Madoff scandal.

[27] At that auction they also sold Judaica from the collection of Jerry Lewis, with items on view at the gallery dating back as far as a Torah binder made in Italy in the 1690s.