I also quoted the statute that very clearly says it's illegal to import paraphernalia, and which explictly lists water pipes and glass pipes as parphernalia. I merely quoted the Posters 'n' Things case because it defines the state of mind (knowledge) that is required for the crime. And yes, it does apply in this case. (Although quite frankly it's practically irrelevant, because everybody knows water pipes are paraphernalia.)
I was harsh on you because you are advising people that it is legal to do things that are in fact illegal. By dispensing advice in this fashion, you risk getting some in serious trouble.
I don't care what some Customs person told you. They probably weren't a lawyer, and what they tell you is meaningless. If you can get a federal prosecutor to tell you it's legal (you won't), you'd have a defense of estoppel by reliance, but they'd still be wrong -- because the statute I quoted, 21 USC 863, unambiguously says:
It is unlawful for any person to import or export drug paraphernalia.
It doesn't say "import as part of a business." Nor does the statute define "import" as "the business of importing." Nor does it say, "It's OK if you're importing just one, or a dozen." In the absence of any such qualifications, you assume the ordinary meaning of the word "import", which is bringing something into the country.
Think about it: If the term "import" was only meant to apply to the business of importation, why would you need this provision? Selling is already prohibited by the language in clause (a)(1) of Section 863; under your interpretation of "import", the prohibition on importation is completely redundant. It's a well-established rule of interpretation that you don't interpret statutes so as to make them redundant.
Now I've already cited the language -- if you want to dispute it, then you cite language (case law OR statutes) that prove your contention.