When I was a kid, I had a few of these in my room and it made it really easy to swap posters whenever I wanted. Now that I have kids of my own, and we homeschool, the need for temporarily hanging posters on the wall has only increased. So I went looking online for these and couldn't find them anywhere.
This is a really simple design, powered by gravity. A 1/2" length of 1/4" vinyl tubing is inserted into each clip through the hole on the top. The weight of the poster wedges it and the tubing inside and holds it in place. You can attach them to the wall with either 3M Command strips, or use #4 or #6 screws. (I haven't actually tested the screw-type designs.)
I hope you find this item as useful as we have. It took me several prototypes to get it right, but I think it works great. Enjoy.