-unfortunately, you failed the exam and are ready to retry it the next year
-you got your hands on material owned by someone else
As you can imagine, there is no right or wrong answer on this. Here are my general thoughts:
–I would not consider using material that is over 1 year old: It might still be good enough but I don’t think it’s worth taking the chance. How much are you really saving?
–If the material is 1 year old or less, I might go for it. I do think it might end up costing you a few % in the exam. It’s not 10% and is perhaps 5% but likely less than that. The question becomes: Are you willing to pay $XXX to increase your score (and the odds that you will pass) by 5%? Only you can answer that question.
I personally would consider doing it, it all depends on how much you value your time. Why? Don’t forget that your main investment in the exam is not the material or the amount you paid to sign up. It’s all of the time and sacrifices that you put into the exam. Once you know how much you’re putting in, you’ll be able to determine what the best thing to do might be.
Don’t Look Back
If you do end up deciding to use material from the previous year, then don’t question yourself later on. This is not a big factor in helping you pass or fail the exam. Everything else, especially having a solid studying strategy  makes a lot more sense.