March 29 (Reuters) - Shares of online retailer Cafepress Inc jumped as much as 17 percent on their market debut as investors continued to bet on the e-commerce sector.

Companies that sell online have seen revenues soar as more and more people turn to the Internet for shopping. People spent a record $37.2 billion buying goods online during the last holiday season, according to online data firm comScore, a number slated to rise.

Internet retail giants Amazon and eBay continue their rapid growth, even as their brick-and-mortar competitors try to bolster their online presence.

Cafepress, backed by venture firm Sequoia Capital, had sold 4.5 million shares in its initial public offering at $19 each, above its expected price of between $16 and $18 apiece.

At the IPO price, the company, founded in 1999, had a market value of about $322.6 million.

Cafepress also offers print-on-demand services, and lets people create their own designs for t-shirts, stickers, coffee mugs and other items and sell them online.

The San Mateo, California-based company saw revenues rise 37 percent in 2011 at $175.5 million, and shipped more than 3.5 million orders.

Also on Thursday, shares of mobile advertising firm Millennial Media Inc nearly doubled in their New York Stock Exchange debut, while those of industrial parts maker Rexnord Corp edged up 6 percent.

Cafepress' shares were trading up 14 percent at $21.65 in mid-morning trade on the Nasdaq.

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