Dear Dan the Man,
This is a common problem. You have no obligation to participate in a discussion that you find fruitless and irritating. Your friend cannot be a very good friend if he persists on this subject despite your obviously unhappy reaction. Also, you have no obligation to the Libertarian Party to fight on every street corner in its defense.
You should tell your friend candidly that you do not want to discuss this subject. If he persists, or takes offense, you should find a more congenial friend. There are people who are not members of the Libertarian Party but who are open-minded about it and willing to listen, even though you may not be able to convert them. If you are receptive to the ideas of other people about politics and policy, you will find some people receptive to yours.
Is a gratuity appropriate when dining at a restaurant buffet? On the one hand, since the server is not taking and filling meal orders, a tip seems unnecessary. Yet on the other hand, the server is not less likely to be underpaid merely because the restaurant offers a buffet, and so a tip may still be expected. And if the restaurant offers menus in addition to its buffet, then the diners are occupying a table that might otherwise be filled by customers who order from the menu, who would presumably tip normally.
If the server brings drinks, should one tip based on the cost of the drinks? What if the server brings only water?
Dear Gratuitously Challenged,
The pay a waiter gets is adjusted by the market to the probability of getting tips. If a waiter works in conditions where tips are unlikely, he will get a higher wage than if he works where tips are customary, other things being equal. If you go through a buffet line and there is no personal service offered to you except handing dishes over a counter, you are not expected to tip. If you go through a buffet line and the waiter seats you; gives you a drink, even if water; and one waiter is assigned to you, you should give a tip. But the tip need not be as large as it would be if you got full table service.