They took their VISA cards (Very Important Sacrifice of Animals) to the Temple in Jerusalem where the transaction took place. The priests (the store clerks) sacrificed the animals on the altar (like scanning your credit card) and the client's sins were charged by faith to the future Messiah's account.
Millions of animals were sacrificed over the centuries, but not one drop of that animal blood could ever wash away sins (See Heb. 10:3-4). So why did God have them sacrifice so many animals? It was an object lesson for them. Without the sacrificial system, they would have never understood why Jesus had to die for their sins.
When Jesus died on the cross, He became the sacrificial Lamb and the sins of the world were poured into His body (John 1:29, 1 Pet. 2:24). When He said, "It is finished," He was declaring that the spiritual debt of sins had been paid. He paid for your salvation, but you must receive Him for the transaction to be completed. To hear the entire message, go to http://www.churchoflivingwater.net/node/23 and click on "What is Finished?"