I do several things, all of which involve customer service. And none of which involve being rude and sarcastic to customers. The principle of customer service is no different whether you're selling online, working in a restaurant, handling customer service on the phone or selling in a shop.
You - and several other posters here - appear to be saying it's perfectly fine to be rude to customers when they ask for a discount.
How you go about interacting with your customers is up to you. My comment is directed at anyone who thinks it's completely fine to be rude or sarcastic to customers who ask for a discount. And yes, shopkeepers who think that DO need to be taught how to suck eggs. If that's not you, then what's the problem?
As for not tolerating this or that in YOUR shop, well, it's your own money you're doing yourself out of if the customer doesn't buy.
Plenty of businesses have gone under because they couldn't provide good service to customers, or were simply rude to them. Search any consumer forum and you'll find plenty of people complaining about it.
Well, I'm sorry to hear that you've been attacked in your shop but that is no reason to treat everyone who walks through the door as a potential attacker. They're the people who pay your wages. No one says you have to put up with abuse. No one should. But I'll repeat, anyone who thinks it's fine to be sarcastic (or perhaps they think they're just being clever and "witty", who knows) to a customer who makes a polite enquiry about a discount deserves to have their business go under.