So what I'm seeing: an 88.69 BTC purchase props the price up .04 BTC, and a sale of .17 BTC dumps the price .03 BTC. What is missing here?
The 'current' price is the last transaction price. If that 0.17BTC had sold for $1 instead, you'd have seen the price drop $4.30 until the next, hopefully sane, transaction.
The price stays relatively even only because a) nobody will buy for more if they don't have to, and b) it doesn't make sense to sell for significantly less (unless you're just dumping 'free' or 'cheap' coins without regard to price)
If you watch mtgoxlive.com, you'll see that kind of thing happening all day long.