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.
it seems you're missing the point a little bit so i'll attempt to clarify.
even if Overstock sales come from "old coins" and therefore are associated with large bitcoin-days-destroyed, as long as there is nonzero demand this can be a good thing for bitcoin by increasing liquidity. for instance, the fact that litecoin has a larger trading volume than bitcoin is one reason i've begun to take it seriously. most bitcoins must be traded off-exchange or not traded at all, and this kind of hoarding does many things, including hurting the general liquidity of bitcoin at the exchange. anything that gets these old coins available for sale is a good thing, as the small demand that is necessary to keep this "exchange inflation" from driving down the price results in a more equitable bitcoin distribution. this process also stress-tests the ticker price against the actually present demand (bitcoin hoarding results in hyperdeflationary events that distort the ticker price).
in other words, it's not the same people who are continuously buying BTC to use at Overstock, but because the demand is present to soak up the "Overstock-inflation", it is still a closed loop that shouldn't affect the ticker price negatively, and generally contributes positively to the health of the bitcoin economy.