Meryl, I can completely identify with the way you must have felt in that situation having worked as a cashier/customer assistance for the last two or three years.
It's the worst to have people come to you that think they're much smarter just because of the job that you're working and then try to rub it in your face, only making themselves look stupider for it.
I used to work at a bookstore and customers would assume I knew nothing about books because I was mostly up at the cash registers so a lot of them would completely discredit me if I offered any help or advice to them about anything.
It's always terrible when you have to deal with rude and ignorant people and bite your tongue because they're in the position of customer and you have to treat them nicely so they can provide money to your employer's business. A lot of people out there will completely take advantage of that situation because they know that they couldn't get away with it on a level playing field where you're both just people.
I find the best way to deal with that sort of thing is just realize that the rude ones are just acting very immaturely and they only make themselves look worse for treating those that provide a service for them poorly.
Oh, the joys of customer service!