Lately I have seen a few items very similar to the way I make some of mine and just had a seller ask to buy a pattern for one of my items. I did not take offense, and actually consider so called "copying" a compliment. I feel that there are enough customers out in the world that we can all share a little success. Most of us get our ideas from other sources, whether it is nature, art, someone else's items, or life. As an individual artist or crafter, it is then up to each of us to take that idea, look at it "outside the box", tweek it, and make it ours. If it works, great, it it doesn't, then go on to the next inspiration.
If someone wants to use one of my items for inspiration, fine. If an items stops selling because the market has become full of identical items, then I would look at my item again to see how I can improve on it and make it stand out from the others. If not, then I would find a new item to create. It helps keep me creative, my mind working, and adds a new challenge.
This pertains to using someone else's product for inspiration. It does not pertain to actually copying, detail for detail, someones else's product, photos and wording. That is going too far, and when that happens the original artist/crafter has a right to be upset and try to put a stop to it.