When somebody pays you to do it they own the copyright to it unless terms are reached otherwise. If DAZ buys a product from me I sign over the copyright. If I sell a product on DAZ then I sill own the copyright. If you sell to a business or a single individual and can be stipulated that they hold the copyright. It mostly depends on what the buyer wants.
Not according to my experience with copyright law. Unless the agreement expressly states they are buying the right to the work as a work-for-hire, and not just paying to have it commissioned for right to use, it is owned by the creator.
For example, in publishing, you see a lot of books that are re-published with a new cover, especially by large houses. The main reason is not to trick us readers into buying the latest Stephen King book that we have forgotten we already bought and read with an older cover, although I’m sure they don’t mind that ... lol. It’s because what they are paying for is for the artist to create an original work to their specifications for their right to use for X amount of time or X amount of copies.
When a new print run needs done, if the art contract doesn’t allow for use beyond what initially agreed to, then the publisher either has to re-negotiate the new fee, which almost as much as the original fee, or hire a new artist.
There is a great explanation that translates all of the legal wording but still makes it fairly clear: http://www.ivanhoffman.com/own.html
Basically, if the contract does not specify work for hire, even if you are commissioned to do it, it’s it remains property of the artist.
I do not do work-for-hire contracts unless that client is willing to pay or the nose for full rights. I find them unfair to the artist. So if they demand that, they pay handsomely or I don’t do it.
So you see why I am both willing to pay for my requests and the modeler still own anything they make for me. Heck, I’ll even advertise for them ... lol.