One more difference:
Adsense always looks like advertising. So even if Google didn't prescribe limits per page, you would degrade your site's quality if you used too much of it.
But Ebay RSS feeds (and widgets and custom API-driven modules) can sometimes be presented as a service of the site: aggregating auctions of interest to the visitor; this is perceived as a site feature rather than advertising.
See for example:
http://AntiquePerfumeBottles.info
See also the sites linked to from the bottom of that page. These mini-sites only have two pages each (the home page plus the Pluck Media display page) and take me about one hour to build per site.
Since January 1, AntiquePerfumeBottles.info has earned $11.45 (from Ebay alone, not counting the Adsense, nor the Pluck Media embedded ads). It contains only three Ebay campaigns:
BIN list: 202 views. 19 clicks @ .22 ea.
Auction list: 202 views. 40 clicks @ .14 ea.
Books list: 201 views. 5 clicks @ .37 ea.
(Some of those other mini-sites have up to 6 campaigns. The Siddur.org site has 13 campaigns. Accordionist.org has 10.)
Most campaigns do not get such a high EPC. 2-cents to 9-cents is actually my most common range. Across the whole portfolio -- currently 206 campaigns and growing -- the average is 8-cents.
Besides the perfume bottles, my highest EPCs are from campaigns related to
- electric lawnmowers
- sports tickets
- tools (sanders, paintguns, etc.)
- playboy memorabilia
- old cantorial records
- collectible dolls
But because Ebay is adjusting EPC to effect a PPA model, the EPC may depend as much on the demographics of the site as on the subject matter of the Ebay feed.
Although the EPC averages lower than Adsense, I get a lot more clicks on the Ebay rss feeds than on the Adsense ads. So they balance out.