secure S3 uploads from third party users

We are using a plugin to create an ecommerce marketplace with wordpress. We would like to store digital products on Amazon S3. The plugin allows third party vendors to upload products, but they store on our server. We are looking for a way to have these store on S3. The problem is that we do not want the third party vendors to be able to access the S3 bucket on thier own and we are concerned about security. Is there an elegant solution for this?