I went to Goodwill today, looking specifically for box springs. (You see, we bought our mattress at Goodwill several months ago and the lack of a boxspring is beginning to be keenly felt.) I found a great many mattresses and box springs lined up against the wall, and, pleased with the selection, fetched an employee and told them I wanted to buy THOSE two box springs. (Our mattress is king-sized, and thus two twin-size box springs are used.)
For the record, the mattresses lined up were rose-floral, blue-floral, blue & green stripes, gold & cream stripes, and a different shade of pink floral. The box-springs I wished to buy were gold, cream, and brown stripes....coordinating with (but not matching) the one mattress.
When a manager at length arrived, she informed me that she would not sell me the box-springs. "They must be sold as a set," she said, indicating the gold/cream striped mattress and one of the box-springs I was intent on buying. I pointed out that the many mattresses and only two box springs indicated that they received single mattresses all the time. "They must be sold as a set," she repeated. She then darted over to the blue-floral (the closest twin-size mattress) and, pointing at the remaining box-spring, said "These two could be a set, too!"
At this point I said that if we couldn't come to an agreement, then we couldn't do business. "I do not want to buy a mattress. I do not NEED a mattress."
Thrift store is right; I ended up spending no money at all. I suppose that's the ultimate in thrift.