I was reading something earlier about the obligation to inform the original owner when reselling a design previously purchased from them and telling them who the new owner was. The below were my thoughts on the subject.
Well, I shall play "devil's advocate" here. Though this doesn't personally concern me as I never sell or trade away any designs that I buy, and still own every single one I've ever purchased.
That said, I can understand the thinking of people who sell or trade away designs without telling the original designer. If you bought a hand-made stuffed animal, and some time later gave or sold that stuffed animal to someone else, would you necessarily remember to contact the person who originally made the doll to tell them this? If you passed by a hand made jewelry shop one day and bought a necklace, and a year later you sold that necklace on eBay, would you remember to go to the shop and tell the person who made that necklace that you sold it, and tell them who you sold it to? Or, more similarly, what if someone bought a physical painting from an artist's gallery, but then a year later they sell that painting at a garage sale before moving. Would that person be obligated to go back to that gallery, find the artist, and tell them that not only they sold the painting, but give the name and contact information of the person who bought it? If you went to a comic book convention and a famous comic book artist sold you a drawing, would you remember to contact that comic book artist and tell them that you later sold that drawing and who you sold it to?
Most people probably feel like once they buy something, it's their's, and they don't need to inform a previous owner of what they do with it.
On the flip side, what if someone you sold a design to told you that you had to inform them of what you spend that money on they paid you with? Most sellers probably would not feel obligated to tell the person who paid them how they spend the money they were paid, as they feel like once paid that money is their's to do with as they please. I'm pretty sure that if some buyer had a list of rules on their profile that included something like "you must inform me when you spend the money I pay you, and tell me what you spent it on and who you paid it to" most sellers would not adhere to that. Similarly, most people once they buy something and become the owner of it they don't feel it necessary (or simply don't remember) to inform the original owner.
Again, I personally never do this as I never sell, trade away, or give away anything I've ever gotten. I'm merely saying that I can understand the thinking of those who do. When someone buys something, they feel like it's theirs. Unless it was lent or rented to them and ownership still remained to someone else, otherwise I'm sure a lot of people feel that once they are designated the "owner" of something they no longer need to maintain connection with the person they bought the thing from.