Nearly four out of five respondents in a survey of online adults made a purchase online in the last six months, Nielsen Online says. Travel represented the biggest category of web purchases, while clothing and accessories were the top retail purchases.

78% of U.S. adult Internet users made at least one online purchase in the previous six months, according to Nielsen Online’s recent @Plan survey, a quarterly survey of 36,000 Internet users 18 years old or older.

The top retail transaction category was clothing and accessories, with 28% of total transactions, followed by books with 26%. 38% of the Internet users made travel-related purchases-the top transaction category overall.

“The sheer convenience of being able to comparison shop from your home or office has become all but irresistible. Possible early concerns about online security have been sufficiently addressed, and consistent on-time delivery and reasonable shipping costs have bolstered consumer confidence,” says Nachi Lolla, research director for commerce at Nielsen Online. “The challenge for retailers is no longer how to lure shoppers online, but how to differentiate their brand among all others.” Inc., No. 1 in the Internet Retailer Top 500 Guide, drew the highest unique online audience for e-retailers in October, with 48.3 million consumers spending on average 22 minutes and 45 seconds on the site, Nielsen says. Wal-Mart Stores, No. 14 placed second with 25.3 million unique visitors who shopped for almost 14 minutes. Target Corp., No. 19, had the third-highest online unique audience, with 23.8 million visitors spending an average of 9 minutes, 36 seconds during the month, according to Nielsen Online.

In October, Nielsen Online found that the average Internet user initiated 59 sessions and visited an average of 105 domains. Web users viewed an average of 2,372 web pages, spending on average 55 seconds per page in October.