The move expands eBay into a fixed-price model, beyond its traditional auctions.
"Half.com's fixed-price format complements eBay's current business by giving our existing users new choices for trading," said Meg Whitman, eBay's chief executive. "Half.com will also attract a whole new group of buyers and sellers to the world's largest trading platform."
The financial terms of the stock-for-stock deal were not announced. The number of shares is estimated to be between 4.6 million and 5.5 million. eBay said the acquisition will result in significant increases in operating expenses for the combined company for the next several quarters.
Half.com is a fixed-price person-to-person marketplace for previously owned products like books, CDs, games, DVDs and videos, with all items selling for at least half off the list price. The company has 250,000 registered users and 1 million items for sale.
The Half.com Web site will continue to operate independently with founder Joshua Kopelman as president, reporting to an eBay executive. Both Half.com and eBay sites will promote the other.
Half.com, founded last year, made headlines in May for convincing city leaders in Halfway, Ore., to change the town's name to Half.com.
eBay rival Amazon.com entered the fixed-price auction market last year when itits zShops area. Unlike Half.com, zShops sellers can offer new goods and can sell them for whatever price they choose. Yahoo, the second leading auction site after eBay, also has a fixed-price marketplace in addition to its auction area.