The "Sell By" date indicates the last day the store can sell it - not how long it will be good. The general rule of thumb is that milk, properly stored, will be freshed if used within 5-7 days after opening - although it may still be perfectly fine for 2 weeks.
Buttermilk, because of it's high acid content, will tend to thicken faster than it spoils. It's just thick, and not big clots of whey swimming in water, it would prossibly be ok.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain