How does "work-for-hire" work? What makes it different from purchasing copyrights?

[newsmike]

56 minutes ago, mabelma said:

Agreed. I personally like to add my copyright terms on my FAQ, to make sure people understand that whilst they hold commercial rights to sell the image, I will always hold rights to display it in my portfolio and on my social media pages as completed work, unless a new agreement is reached and that has an extra cost.

Absolutely correct. Your written agreement always supersedes what someone thinks the "global rules" are. 

[mabelma]

23 hours ago, newsmike said:

Absolutely correct. Your written agreement always supersedes what someone thinks the "global rules" are. 

Exactly! Be as clear and direct with what you want to happen with your work, and things will go much more smoothly with your buyers.