Q: How do the rights work? Do you own them and individuals can purchase rights to use them?
A: I always maintain ownership of everything I produce. When I am commissioned for a project that party gets image usage, my fee, and post processing folded into one rate. If others would like to use the images, they pay a smaller fee for usage only. If it is within a year of of the original shoot, I will ask the commissioning party if they mind if I give usage to another. This party is generally a partner on the project.
To simplify things as much as possible, I make my image rights open ended; the license holder may use the images for in house purposes as they choose for as long as they choose. This covers web, social media, print advertising and general media usage. It does not allow for sharing among construction partners, designers, friends, and multiple entities under an umbrella corp.
My preferred scenario is the multi client model, where everything is done up front and everybody benefits. For each additional client, I add a half day fee to cover usage only, because post and shooting will only be done once. The bill is then split between the number of clients and everybody gets their own set of images with usage. The more clients the lower the bill for all.