It is not against the terms of service for either eBay or Amazon to have ads from the other. However, there may be limits to the number of ads altogether.
You have to also consider Google. Having too many ads with respect to content can lower your Google ranking, especially if you have too many ads above the fold. To increase your ranking, keep your Amazon and eBay ads below the fold. That means people see content first and they see the ads after scrolling down. This also helps keep visitors on your page longer, which also increases rank.
I know I told you more than you were asking. But it's all important for having a successful website, no matter if it's articles on HubPages or content on your own site.