
<PAGE>

                                                                     EXHIBIT 5.1
                           OPINION OF COOLEY GODWARD

                       [LETTERHEAD OF COOLEY GODWARD LLP]

September 30, 1999

eBay Inc.
2125 Hamilton Avenue
San Jose, CA 95125

Ladies and Gentlemen:

   You have requested our opinion with respect to certain matters in connection
with the filing by eBay Inc. (the "Company") of a Registration Statement on
Form S-3 (the "Registration Statement") with the Securities and Exchange
Commission (the "Commission") covering the sale by certain selling stockholders
of 15,931,734 shares of the Company's common stock (the "Selling Stockholder
Shares").

   In connection with this opinion, we have examined the Registration
Statement, the Company's Certificate of Incorporation and Bylaws, each as
amended and such other documents, records, certificates, memoranda and other
instruments as we deem necessary as a basis for this opinion. We have assumed
the genuineness and authenticity of all documents submitted to us as originals,
the conformity to originals of all documents submitted to us as copies thereof,
and the due execution and delivery of all documents where due execution and
delivery are a prerequisite to the effectiveness thereof.

   On the basis of the foregoing, and in reliance thereon, we are of the
opinion that the Selling Stockholder Shares are validly issued, fully paid and
nonassessable.

   We consent to the reference to our firm under the caption "Legal Matters" in
the Prospectus included in the Registration Statement and to the filing of this
opinion as an exhibit to the Registration Statement.

                                          Very truly yours,

                                          Cooley Godward llp

                                          By:       /s/ Karyn R. Smith
                                             __________________________________
                                                      Karyn R. Smith
