This was one of the questions asked today on a board I frequent. I thought it a very interesting slice-of-life question. So I looked it up. And my first purchase was November 25, 1998, for three books (Dirk Pitt Revealed, Airframe, and Contagion). I remember making that purchase, and I think I still have those books.
Then I looked at 1999 purchases, of which there were only five. I picked up the pace considerably in 2000—with three pages of orders—and apparently never looked back! ;D