If someone offers "Money Back Guarantee" for any type of service or product, they usually have a set of rules that you have to abide by. If you haven't violated any of those conditions, then they WILL give you a refund - if they don't, it's false advertising and that opens a whole new can of worms in the legal system.
Just be sure to read the terms and conditions. I use STEAM for some games that I purchase. Various games have different restrictions usually based around playing time. If I play for longer than 10 hours then, no refund available, or partial refund available.
The local book store has a policy that they will give a refund for up to 30 days as long as you provide a receipt and there are no obvious defects to the book (pen marks, coffee stains, dog-eared pages etc).
As long as you follow the rules, any company that says they offer a refund, will honor that refund. It may not be an INSTANT refund, or it may be a credit - so again, read the terms.