Slightly off topic, but not by that much, one 'scam' that some prospective commercial clients (some, not all) do is try to get photographers to work for free.
There are so many trying to make it (digital photography has been a great democratizer), some that do have nice portfolios, that they fall for the 'we can't pay you this time but we will pay you next time (never happens as they move on to the next sucker) and/or it will give you great exposure".
Numerous times I have been approached to give for free images to be sold at charity auctions. With the promise that the exposure would be worth it. I saw what was happening (it would always be some lackey working for the rich persons' 'charity') and never fell for it because I made sure to not rely on my photography as my primary source of income.
But desperate artists do.
I also knew of several gallerists who would give an artist a show as long as that gallerist could keep a selection of their work for free, not matter if they sold any pieces. Some would have group shows and then select what they would keep from all the artists!