S3 Bucket Transfer to another account

No, you can't transfer the ownership of S3 buckets across accounts. Once bucket is created in an account, it'd be owned only by that account through out it's life.

However, you can copy the data from these supplier buckets to actual owner account buckets.

I'd suggest you to go through this storage blog, which covers all the copy options and their pros/cons.

Batch replication is most fit for large volume of data copy across buckets. Refer Batch Replication Document. Refer batch copy permissions setup if you'd like to copy through S3 batch copy.

Hi, you can't transfer the ownership of an existing bucket to another account, But you can copy its accross 2 accounts in another bucket owned by the destination account.

See https://docs.aws.amazon.com/prescriptive-guidance/latest/patterns/copy-data-from-an-s3-bucket-to-another-account-and-region-by-using-the-aws-cli.html