From http://edgar.sec.gov/Archives/edgar/data/1130713/000113071315000018/ostk-20150331x10q.htm ...

All customer Bitcoin payments are converted 100% to dirty, filthy fiat at transaction time:

At present we do not accept bitcoin payments directly, but use a third party vendor to accept bitcoin payments on our behalf. That third party vendor then immediately converts the bitcoin payments into U.S. dollars so that we receive payment for the product sold at the sales price in U.S. dollars.

And while they hold some coins, it's ... uh ... not workin' out all that well for them:

We hold cryptocurrency-denominated assets such as bitcoin and we include them in other current assets in our Consolidated Balance Sheets. Cryptocurrency-denominated assets were $233,000 and $340,000 at March 31, 2015 and December 31, 2014, respectively, and are recorded at the lower of cost or market based on an average unit cost ... Losses on cryptocurrency holdings were $117,000 and zero during the three months ended March 31, 2015 and 2014, respectively.

They do not appear to have increased their holdings in Bitcoin at all. Unfortunately, it doesn't seem as though they give any indication of BTC transaction volume in their quarterly filings.