What new economic practive of the 1920s was credited with being responcible for people buying more?

I'd probably lean towards "installment plan". As the number of stores grew across the nation and more products became available, they began looking at different ways to get people to buy more even if they didn't have all of the cash at the moment. So as one store offered up an installment plan to help pay off that new car, fridge, or whatever, other stores were forced to compete allowing more people to get even more stuff. It's a tricky question because there were also early credit cards available at the this point in time, but installment plans are based on roughly the same concept - allowing people to buy on credit with a promise to pay everything off at a later date. As I see it, though, instead of having to sign up for a specific card you could walk into any store with an installment plan and get what you wanted