If you're a popular mobile app developer, don't be surprised if eBay (Nasdaq: EBAY) clicks your "Buy it Now" button.

The company behind the namesake auction site and the faster growing PayPal financial transaction giant is acquiring Critical Path for undisclosed terms.

Portland-based Critical Path develops smartphone applications for Apple's (Nasdaq: AAPL) iPhone, Google's (Nasdaq: GOOG) Android, and Microsoft's (Nasdaq: MSFT) Windows Phone 7 platforms. More specifically, it's also the company behind eBay's own eBay.com, StubHub, Shopping.com, and eBay Classifieds apps for the iPhone.

If eBay buying a mobile app specialist sounds familiar, it's because the company also snapped up the maker of the RedLaser barcode scanning application this summer. With nearly a million active users of RedLaser at the time, it was easy to dream out loud. Would eBay be incorporating auction listings into RedLaser the next time someone scanned a retail UPC to do a little comparison shopping?

This time around, there's little left to the imagination. Critical Path has been working with eBay for two years to make many of its mobile programs. The relationship will now expand, and eBay will be able to make sure that Critical Path doesn't crank out smartphone interfaces for competing websites.

Competing websites? Sure, eBay is the runaway leader when it comes to auctions, but eBay Classifieds naturally butts heads with Craigslist, StubHub is facing beefed up resale efforts from LiveNation's (NYSE: LYV) Ticketmaster, and Shopping.com has long list of rival comparison shopping sites.

For investors fretting over eBay's uninspiring growth on the marketplace end, the mobile boom creates an opportunity to get excited again. Yes, folks, there's more to eBay as a growth story than PayPal now.

What should eBay buy next? Share your thoughts in the comment box below.

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Longtime Fool contributor Rick Munarriz is a satisfied eBay user with 178 positive feedbacks to show for it. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.