Car-sharing service Zipcar Inc. changed the car-rental business, but its hourly rentals to young and urban customers proved a lousy business.

The Cambridge, Mass.-based company signed up more than 760,000 members since its founding in 2000 but struggled to post a consistent profit. On Wednesday, Zipcar agreed to sell itself to Avis Budget Group Inc. for about $500 million, or $12.25 a share, well above its recent trading price but below its initial-public-offering price.

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