"Off the rack gives me hives."
-- Cordelia, "Out of Mind, Out of Sight"
The Scoobies are used to being personally affected when demonic nasties come to Sunnydale. And they're used to strange things happening at Sunnydale High -- after all, the place is built over the Hellmouth. But they've sealed the Hellmouth, so they don't think anything's out of the ordinary when items to be sold at the first annual band fund-raising rummage sale are stored in the school basement...which, one might recall, is directly above that Hellmouth.
Once the rummage sale begins, it's clear the stuff for sale is far from ordinary. People seem to be strangely affected by the items they're buying -- things from undemonic homes, donated by undemonic people. So the reactions these items are producing are, to say the least, unexpected. Willow's acting weird. So's Xander. And even Buffy's mom bought something....
The situation quickly spins out of control, and the school has to be quarantined...leaving Buffy and Giles to sort things out before whatever demon this is sells them down the river.