They're not talking about sending billions, just a reasonable portion of their income...comparable to what the average american pays. Buffet calling up gates and saying "hey, you want to do 30% this year?" would not have as significant impact on revenue. Buffet said he paid about $7 million last year, 17.4% of his taxable income. Double that and it's a revenue increase of $7 million. Say Gates makes the same, then it's $14 million. What are total tax revenues? $2.1 trillion. So Buffet and Gates pact would result in a about a one thousandth of a percent increase? Not significant.
Now, Buffet could make a lot of phone calls to his buddies and get a lot of pledges to agree to this new voluntary tax rate. A large enough number of agreements and if things were organized well, would also take care of the collective action issue. But then why do we need government at all? Let's just all form rules and agreements on our own. Heck, we could even write up a constitution to solidify it all. Have a system of representation. We could call ourselves...oh, I don't know...the United States of America.