No, it isn't eBay's fault.
99% of online forms automatically adjust to the user's screen settings. But some don't - this happens to me sometimes. It's very frustrating.
If you can't pull the bidding page up so that the button comes into view you may have to temporarily change your screen settings to a lower resolution. On a Windows computer, right-click anywhere on the desktop and go to the display settings. You can change it back afterwards, but it may alter the position of your desktop icons!