I disagree with Filippo, the first sentence can make sense, but there is probably a complementary sentence before it, and it's written in a weird way. The key point is that the word "American" there is an adjective, not a noun. What kind of 'need'? An 'American need'. Here's a hypothetical prelude to that sentence: "(There are many American needs: the American need to buy, the American need to be famous, the American need to hustle. And) Just as American as the need to buy, social observers say, is (the American) buyer's remorse." For the other two questions, you'd need to post the full sentences, because it's hard to see the meaning without context. I can only hope that made things less confusing

"The backlash...dates nearly to its conception." "nearly" means "almost," "to date" is to say how old something is, like Filippo said: "The bone dates back to 500 years ago." It's common to hear it in the passive voice: "These documents are dated 4 years ago." "Our ambivalence and need... come in slick, packaged form." "slick" is an adjective, it's a bit hard to define, something with a cool design, like a sports car or a perfume box. "slick" and "packaged" are a sequence of adjectives. You can line up as many adjectives separated by commas before a noun , but it will sound like you are selling something: "It is a powerful, discreet, stylish, lifestyle-enhancing, robust computer."

This sentence : "Just as American as the need to buy, social observers say, is buyer's remorse." , does not make sense. Maybe this sentence should be " ANY AMERICAN FEELS THE NEED TO BUY, BUT AS EVERYONE KNOWS, SOMETIMES YOU EXPERIENCE A BUYER'S REMORSE". In other words you are sorry you bought something. This sentence: "dates nearly to" it is also a bit of a problematic sentence. For example... you found a bone on the street. If you give it to an archeologist he will say THIS BONE DATES BACK TO 500 AGO OR NEARLY 500 YEARS. That would be the correct use of the sentence. This sentence : "come in slick" means absolutely nothing. I don't understand the meaning of the passage you posted. Sorry