Illegal, pirated software abounds on the auction sites. First and foremost, read the auction description carefully. If the description isn't adequate or seems vague, email the seller for clarification. These aren't all must-haves, but their presence does lend credibility to a software auction. Most sellers of pirated software aren't up front about it.
Dutch auctions can raise eyebrows, too.
Although it's certainly possible someone has 50 legitimate copies of Microsoft Office to unload, it's also possible they stayed up last night burning 50 copies of one disk. Lastly, before bidding on any auction, but especially software auctions, check the feedback others have left about the seller. Don't just note the negatives or positives given. Go ahead and read the actual feedback postings. If the seller doesn't have a proven track record, you might want to look elsewhere.