Can I share completed files from one seller to another seller to complete a new order?
As a buyer?
Yes, I wouldn’t see why not. As far as the second seller’s concerned, you’re just sending them a file - they don’t need to know who created it etc.
As a proofreader, I have proofread content for other sellers. I have corrected documents such as their resumes, blog posts, and website content. There has never been an issue.