Move files directly from one S3 account to another?

Yes, there is a way. And its pretty simple, though it's hard to find it. 8)

For example, suppose your first account username is [email protected] and second is [email protected].

Open AWS Management Console as acc1. Get to the Amazon S3 bucket properties, and in the "Permissions" tab click "Add more permissions". Then add List and View Permissions for "Authenticated Users".

Next, in AWS IAM (it's accessible from among the console tabs) of acc2 create a user with full access to the S3 bucket (to be more secure, you can set up exact permissions, but I prefer to create a temporary user for the transfer and then delete it).

Then you can use s3cmd (using the credentials of the newly created user in acc2) to do something like:

s3cmd cp s3://acc1_bucket/folder/ s3://acc2_bucket/folder --recursive

All transfer will be done on Amazon's side.

Use the aws cli (I used ubuntu 14 ec2 instance) and just run the following command:

aws s3 sync s3://bucket1 s3://bucket2

You will need to specify the account details for one, and have public write access or public read access to the other.

This will sync the two buckets. You can use the same command again later to sync quickly. Best part is that it doesn't seem t require any bandwidth (e.g. files are not passing through local computer).