Amazon.com Inc. is investigating suspected data leaks and bribes of its employees as it fights to root out fake reviews and other seller scams from its website.

Employees of Amazon, primarily with the aid of intermediaries, are offering internal data and other confidential information that can give an edge to independent merchants selling their products on the site, according to sellers who have been offered and purchased the data, as well as brokers who provide it and people familiar with internal investigations.

The...