In the 1920's a method that enabled people to buy goods they could not afford was

The correct answer is: "borrowing funds".

The 1920s were a time of low interest rates, when borrowing money was relatively cheaper than in former periods, so it was common that citizens asked for loans in order to face the payments of the goods and services they desired, in case they did not have enough money of their own to pay for them.

The borrowing mechanism were even applied to the purchase of stocks. Such practice was refered as purchasing stocks on the margin.