Is it ethical to purchase an item that you have every intention of returning

It depends on your intent.

If you bought it intending to keep it, but upon using it for a few days discovered that it was defective, didn't function as advertised, or is not fit for purpose, no. That's a valid reason to return it and request that the situation be made right, and you had to try using it to find that out.

If you bought it with full knowledge that you intend to "borrow" and return it, yes. If you only need something for a short time, look for someone you can borrow or rent one from. Yes, that might be a little more effort, and it might actually cost something, but it's the right thing to do. With that practice, the store is losing money on the "open box" item, and essentially, every other customer there is now paying for whatever you were doing. And because of you, people with valid reasons to return something have to fight over "restocking fees" and the like.