That sort of scheme would certainly not past the smell test. Buying some already-used clothes that had some sort of historic or museum-quality value (G. Washington's riding breeches) might very well qualify, but trying to purchase new clothes claiming they are an "investment" is a silly justification and would no doubt not be honored by any seller of same since it would be that seller rather than you who would be liable for the tax should it come to that. It does not matter that YOU "consider them an investment" at all. I doubt that you've been lied to but it may be that whoever told you that didn't grasp your little "consider it as an investment" trick.
Keep in mind you must first buy them as new and at that point they have no established value other than their retail value (with 23% of that being the Fair Tax). The stunt of "calling everything as an investment" is clearly beyond the pale and I'd think you'd realize that if you had read the FairTax bill or even spent a bit of time on the FairTax website. You can "call" it anything you wish. Not paying a normal applicable retail sales tax on it is another thing entirely.
In fact, even common sense should tell you that.
Many museums have costume collections. No, that term doesn't have anything at all to do with Halloween costumes or things worn on a play of movie. It isn't only famous people whose clothes are in museums. Places such as the Met in Manhattan and The Chicago Historical Society gladly accept clothes from not so famous people. And there are also museums around the world, whose collections are just on designer or another. Ergo, buying designers clothes and not just the haute couture, is therefor an "investment".
I pulled no "trick"; just talked about a lifestyle you know less than nothing whatsoever about.
If something is an investment, then perforce, according to the fair tax "rules", it is untaxable.
Did you know that some people collect barbed wire? Some collect beer cans. You'd be quite surprised what these things fetch, I bet. Almost ANYTHING can be called an "investment", because someone collects it !