When I see stuff like this, I always think "feature" not a company. There are two directions you can take this to avoid the "but its just a feature" issue - a) build it so that you can license the technology to people that have made a company out of selling domains (domainsbot.com is a great model to take a look at) b) become a reseller or registrar so that you can fully deliver the service and collect the recurring…
The rationale being that you initially build some specific functionality you can't find in the market (feature), over time rounding this out into a product and finally evolving into a business as you understand how to monetize it and where opportunity for growth lies.
You don't need to have your business totally conceived on day one. And in fact, finding where the business lies (or whether you even want to grow your idea into a business) is something you'll have a much better understanding of after a few months of being out there.
Personally (disclaimer: I run the domain search site Domize - http://domize.com) I think it's wonderful we can launch these "just a feature" websites and evolve them into products or businesses over time (or not). These 'better mouse-trap' sites are a fantastic, low-risk way of generating passive income and if you can string a few together you can potentially get to a stage where you can live off them. At the very least, you've demonstrated the kind of initiative and creativity that will provide you with a great talking point on your resume.
Let's not forget, both YouTube (embeddable video for eBay auctions) and Twitter (group SMS updates) started as "just a feature" and evolved into billion dollar businesses.