Younger shoppers are spending less and getting thriftier. Perversely, that's bad news for this thrift store operator.Winmark (WINA -0.72%), the innovative retailer of used toys and clothing best known for its Once Upon a Child and Plato's Closet stores, is seeing some stock selling this morning as its shares tumbled 2% through 12:45 p.m. ET.And why is this happening? It's never 100% certain, but my hunch is that investors may have been spooked by a recent article in The Wall Street Journal, which reported o ...