Computer forensics are playing a key role in the probe of the Virginia Tech gunman, with investigators revealing he bought ammunition clips on eBay designed for one of two handguns used to kill 32 people and himself.

The eBay account and other Internet activities provided insight Saturday into how Seung-Hui Cho may have plotted for the rampage, including the purchase of several empty ammo clips about three weeks before the attack.

EBay spokesman Hani Durzy said the purchase of the clips from a Web vendor based in Idaho was legal and that the company has cooperated with authorities. Attempts to reach the Idaho dealer were unsuccessful.

''Within 24 hours, after Cho's identity was made public, we had reached out to law enforcement to offer our assistance in any investigation,'' Durzy said.

Authorities are also examining the personal computers found in Cho's dorm room and seeking his cell-phone records.

Cho, 23, also used the eBay account to sell items ranging from Hokies football tickets to horror-themed books, some of which were assigned in one of his classes.

A search warrant affidavit filed Friday stated that investigators wanted to search Cho's e-mail accounts, including the address Blazers5505@hotmail.com. Durzy confirmed Cho used the same blazers5505 handle on eBay.

Virginia State Police spokeswoman Corinne Geller said investigators are ''aware of the eBay activity that mirrors'' the Hotmail account.

One question investigators hope to answer is whether Cho had any e-mail contact with Emily Hilscher, one of the first two victims. Investigators plan to search her Virginia Tech e-mail account.

Experts say that when the subject of an investigation is a loner like Cho, his computers and cell phone can be a rich source of information. Authorities say Cho had a history of sending menacing text messages and other communications—written and electronic.

On March 22, Cho bought at least two 10-round magazines for the Walther P22. A day later, he made a purchase from a vendor named ''oneclickshooting,'' which sells gun accessories and other items. It appears that he bought three Walther P22 clips in that purchase, but the seller could not be reached for comment.

Cho sold tickets to Virginia Tech sporting events, including last year's Peach Bowl. He sold a Texas Instruments graphics calculator that contained several games, most of them with mild themes.

''The calculator was used for less than one semester then I dropped the class,'' Cho wrote on the site.

He also sold many books about violence, death and mayhem. Several of those books were used in his English classes, meaning Cho simply could have been selling used books at the end of the semester.

His eBay rating was superb—98.5 percent. That means he received one negative rating from people he dealt with on eBay, compared with 65 positive.

''great ebayer. very flexible,'' the buyer said of his Chick-fil-A Peach Bowl tickets, which went for $182.50.

Andy Koch, Cho's roommate from 2005-06, said he never saw Cho receive or send a package, although he didn't have much interaction with the shooter. Students can sign up for a free lottery on a game-by-game basis, and the tickets are free.

''We took him to one football game,'' he said. ''We told him to sign up for the lottery, and he went and he left like in the third quarter, and that was it. He never went again. He never went to another game.''

Cho sold the books on the eBay-affiliated site half.com. They include ''Men, Women, and Chainsaws'' by Carol J. Clover, a book that explores gender in the modern horror film. Others include ''The Best of H.P. Lovecraft: Bloodcurdling Tales of Horror and the Macabre''; and ''The Female of the Species: Tales of Mystery and Suspense'' by Joyce Carol Oates—a book in which the publisher writes: ''In these and other gripping and disturbing tales, women are confronted by the evil around them and surprised by the evil they find within themselves.''

Books by those three authors were taught in his Contemporary Horror class.

Experts say things like eBay transactions can be hugely valuable in trying to figure out the motivation behind crimes.

An examination of a computer is ''very revealing, particularly for a person like this,'' said Mark Rasch of FTI Consulting, a computer and electronic investigation firm. ''What we find ... particularly with people who are very uncommunicative in person, is that they may be much more communicative and free to express themselves with the anonymity that computers and the Internet give you.''

Cho's computer could hold a record of just about anything he has done, even of activities or communications he may have tried to erase. But Rasch said that likely will not be a problem, noting the way the gunman created a record of his thinking in videos, photos and documents.

''This guy wanted to leave a trail. He wasn't trying to conceal what he did,'' Rasch said.