Does offering guarentees increase sales?
It depends on how you look at things. If you target a market that has grown to expect such guarantees, and you don't offer them, your sales might suffer. If you do offer them, you'd be on par so guarantees would basically not be comparison criteria for the customers. If you push the envelope and go for say a 90 days moneyback guarantee, you might win some extra customers. Unfortunately you might attract more people looking for a free ride as well.
If you target the local market, and virtually none of your competitors offer an uptime guarantee (such a bad name for it), being the first to offer it can give you an important edge, especially in the eyes of a customer who does not understand what it really means.
Guarantees, testimonials, nice web design everything can and will help, more often than not, for no good reason. This is why some love marketing, and some hate it.