The way I see it, the ecosystem cycles through buying Bitcoin to be spent at Overstock, and Overstock selling it back to someone else, and then someone buying or using Bitcoin to spend it back on the site again. Overstock's circulation is very good for Bitcoin, and I highly doubt its hurting it. Its causing more buys than hurtful sell pressure.
Like others said, it is certainly speculative by the looks of what is currently draining the price.
I'm curious what makes you think this is happening? It doesn't really make sense for the lay person to buy goods on Overstock.com in this manner. Why bother converting your dollars to bitcoin, then buy them back once you've spent them? That item will now have cost more after the bitcoin exchange fees. It's just a more expensive hassle that most people don't want to fool with.
I think the majority, if not all, sales at Overstock are from the early adopters who acquired a lot of coins.