According to the Guardian:

“Because both are scared of Google, which not only dominates search, but is launching a payments system and plans to take over the world’s classified advertising“.

They make a good match. As the article points out, the core of the deal centers around advertising distribution and, most interestingly, PayPal.

I think Google will certainly consider that partnership a threat, and an opportunity for strategic advantage:

“More than one analyst noted, however, that Mr. Schmidt’s denial left a lot of room for a Google service that could cause significant damage to eBay or PayPal, or both….Google Base has the potential to become a significant threat to both PayPal and eBay if Google expands the coverage and features that it offers..“

So, not a case of “if”, but a case of “when”….