Picture First
The picture is at the left on both sites because people usually read from left to right. And producers want the consumers to see the most attention-grabbing element first, and picture is better than words in attention-grabbing.
Button Placement
Amazon Buy Button is at the right, signalling that when you click this, you are done on this page, you are going forward.
But Ebay on the other hand, bidding something doesn't mean you are done, you just placed a bid, you still need to keep watching, and maybe modify your bid later. And often time, you would want to review any info on the product page before you modify your bid accordingly.
Promotion Placement
They are scattered, because they are different kind of promotions targeting different audience. The one under the image is for sellers, while the one under the bid box is for buyers.
Logic
You are right, there got to be some kind of logic, and you are not detecting it because you are not actually the user, you are not bidding something yourself (assumingly), you are just studying the design of the website. But for a bidder, this layout is logical.