Yes, of course I can't literally guarantee a 700+ score! That's why you could potentially end up failing in your goal, but getting all your money back. The guarantee isn't technically the score itself--it's the promise of a full refund if you don't achieve a 700+ score.
This is not the right decision for everybody, that's for sure...especially since in order to get the guarantee, you would have to pay a multiple of my regular rates, which is dependent on your probability of scoring 700+. The vast majority of my students choose to simply pay my regular hourly rate, and take their chances.
And yes, there is a high degree of trust involved. But this is an issue that could be solved by an escrow service, for example.
One point I would disagree with, though, is that an individual private tutor is less trustworthy than a large company. I have known large companies like Verizon and Comcast to be far worse, in my 38 years of life experience, when it comes to trustworthiness, than small business owners and mom-and-pop shops.
Let me put it this way: my name is my business, and my business is my name. I have built an empire of trust around one person and one GMAT tutor: me. I'm not an anonymous worker who can hide behind a company logo. If I were not keep my end of the bargain in this type of situation, then it would give my business a bad name, and I can't recover from that, so this fact (in addition to my own strong principles) prevents me from taking advantage of students and clients.
If they score a 680 or 690, then yes, I would still give the money back. But part of the agreement is that you have to take the GMAT at least 3 times.