I wouldn't "copy" or even come close to copying something from etsy for sale. If I got inspiration from something I saw there, it depends on the type of inspiration. If it was like someone had made a type of wooden elephants that sparked a mental train of thought that ended in me making woolen sewn elephants, I would have no issues selling it. If it were something like, Boojboo who is the first person I've seen to put darts in an apron, I'm not sure. Its genius. Is it her idea only? certainly in some ways no, but I'd also feel like I was sort of encroaching on her territory. It depends. If I took the darts idea and used it on my own aprons that were radically different than hers, probably I'd sell them. Afterall, darts in clothing are a widespread thing. I wouldn't sell aprons that were very similar to hers though.
Now, for personal use, if I decided I wanted to try to make something like something I saw on etsy, I would have no qualms at all. I wouldn't copy the exact thing for example trying to use the exact fabric and trying to copy the exact shape, but trying to replicate the shape, or being inspired by fabric with cupcakes on it, well, in my opinion its fine. I have a serious case of "I can make that"itis, and I don't think its a problem to try to make things myself. If I were selling things on etsy, I would have no problems with people trying to make things like mine for themselves, and glad they were inspired by me. I'd only have issues if they tried to copy my designs for sale.