I think the big thing is where the shells and feathers came from. If the feathers and shells came from an animal that was finished with using them than they are not exploitive. the thing about veganism is not the animal product itself but the exploitation of the animal. There is nothing exploitive about walking down the beach and finding feathers and shells.
Agreed. Same with second hand objects.
The topic that's always debated is when and if your use of the second-hand or found item serves to promote it.
For example, say I find a leather jacket in the trash, clean it up, and wear it. Does wearing it encourage other people to buy leather, which might be new? What if it's faux leather but it looks real? How do you draw the line between private use and promotion? If just one person sees me wearing it, am I promoting it? How responsible am I for that potential effect?
Home decor is like clothing. People might see it in your house, like it, and buy something similar but not cruelty-free. Obviously, that depends on the people. But how can you predict who will do what? If you tried to, would you be judging too much? Making the wrong kinds of assumptions?
Vegans/vegetarians have different views on all of this. Obviously, your personal need for the item matters too.
Usually, people decide where they stand on these issues and then get ready to talk about it with other vegans. It does come up, but, in my experience, most people are tolerant of others' differing viewpoints as long as there is a rationale for it.