Actually that’s not really true: I suppose ads sometimes contain information about a product. But insofar as there are various products of similar spec, ads say nothing about which is better. Google searches do.
Ah, but this is my point. How can I distinguish between someone like you who’s (hopefully!) not a scammer, and an actual scammer? You’re all promising the same thing…
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Not exactly. A scammer will offer to pull some strings, or pay people, or put up other webpages, or the like, to influence Google. A legitimate optimizer like (presumably) rfgdxm will actually change your webpage itself in such a way as to make it more appealing to the search engines. He gave us a few examples of the sort of changes he would make, but obviously he can’t tell us everything for free.
Google rankings tell you something about which one is better known, or discussed more often. For example, if Microsoft announces a new product at the same time an obscure little company announces a competing product, Microsoft’s is almost guaranteed to be much higher in the rankings, even if the other one is better. In fact, this will probably be true even if Microsoft’s isn’t out yet, just because more people will talk about it.
Probably few. However, SEO is about getting the extra few to visit the site. In the e-commerce biz, a few extra sales aren’t a problem. 
That you can’t tell is the magic.
It mostly comes down to legit sellers selling the same thing at different prices. E-commerce is mostly done with credit cards. Any scammer who didn’t deliver would quickly have their bank close their merchant account. With SEO, the idea is to get an actual legit seller to the #1 spot on a search. Many people don’t price shop. Hopefully they won’t notice the company in the #9 spot on Google is selling widgets cheaper.