Overstock.com is a legitimate online retailer. The company does not have brick and mortar stores like Target or Wal-Mart, but it is a legitimate company when it comes to Internet shopping.
To some degree, determining whether a company is legit is up to the perception of the person asking the question. Overstock.com is publicly traded on the stock market; as of December 2014, it carried a share price of around $20 per share. The company attempted to set itself apart from other online retailers earlier in 2014 by being one of the first companies of its size and stature to accept bitcoins as payment for purchases.