I offer a money back guarantee, which means that at the time of purchase, all pieces of the unassembled item are presumed to be present unless otherwise noted, and free from defects. The item is also guaranteed to function properly, important for anything that plugs in or uses the science of mechanics like bicycles.
A Money Back Guarantee simply means that you should be satisfied with your purchase or your money back. They are typically offered by the seller, ie., the retail outlet.
A warranty, on the other hand, guarantees that your product will last for a certain number of days, months, or years, and is normally issued by the manufacturer at no extra cost, however retailers do offer them also, but usually charge extra for them, and many of them are services provided by third parties. Retailers also have become creative in marketing them under different names, such as service plans. Originally called "extended warranties", they've gotten a bad rap over the years as not worth the extra cost.
Consider the options, the length past the manufacturer's warranty, ease of making a claim, and what is offered if the item cannot be fixed. Do you get your money back in a pro-rated amount or do they provide you with a brand new item? Do they send your device off to another location for weeks at a time, or fix it at the store on the spot? And do they provide you a loaner while you wait if it is shipped out?
Really it boils down to each retailer's plan specifically and what it covers and for how long. Some even go as far as covering accidental damage such as coffee spills and lost or stolen scenarios. Popular with furniture, cell phones, other electronics, accidental damage can be a worthy add-on if the fine print doesn't exclude every possible cause you may encounter.