When most people think of money back gurantees they think of being able to return an item for any reason within a set amount of days. This is seldom the type of guarantee present in the screen used prop collecting universe.
Many dealers offer a money back guarantee if one can prove the item is not authentic and as described. While this sounds good in theory in practice it means the item has no guarantee as one must convince a seller who has money in hand that an item they already sold is not as described.

The burden of proof is on the collector not the dealer. And if a dealer says its as described what is a collectors recourse? Unless the dealer divulges their source and pratically hands over all the information about the item in question including the original purchase price a collector is not given the details necessary too seriously explore if an item is not authentic.

So just what value besides a marketing ploy does a money back guarantee for authenticity reasons have for the average collector.
If the item is so obviously a fake that a quick question to anyone in the know will reveal it to be a fake then the dealer is probably either not careful or as ethical to begin with.
If the item is more iffy then its hard to reach a consensus on authenticity via pictures alone which can be shared with other collectors. Without a consensus pressuring a dealer refunds usually will not happen.

I've dealt with people who have been proven to be quite questionable, offering a lifetime guarantee. You could tell right away that there was essentially no guarantee at all.

That said, I offer a money back guarantee for 90 days and would certainly work with anyone reasonable for years after the 90 days if there were a true breach in authenticity. I've been working with celebrities and agents for 20 years and doing memorabilia promotions for marketing purposes almost as long. You're either in this for the long haul or you're not.

It is pretty easy to see who's guarantee means something and who is shady early on in a relationship.

Even with someone reputable the lifetime guarantee is only up to their financial ability to honor it. So if a lot of a dealer's stuff is found to be not authentic 10 years from now there is a good chance they will be struggling to stay in business and won't have the means to provide refunds also. The thing with props and wardrobe dealers is most haven't been around all that long so the guarantee thing long into the future is not really so well tested. If the guarantee is for authenticity reasons only the burden of proof falls on the buyer, but the thing is if the average buyer knew how to authenticate things and who to contact from movie productions for such they could largely buy everything directly and avoid the dealer middle man. Few know how to do such.