They are seizing shit without knowing what it is. If you buy bulk from best-feel.com, etc. they will seize it.
I have immense knowledge of organic chemistry and bench science and the discovery of novel fentanyl analogs isn't even remotely easy. GC/MS testing may only show the fentanyl precursor, N‐phenyl‐1‐(2‐phenylethyl)piperidin‐4‐amine. You can extrapolate from GC but you need to do quadrupole time‐of‐flight high resolution mass spectrometry, matrix‐assisted laser ionization Orbitrap mass spectrometry, nuclear magnetic resonance spectroscopy and infra‐red spectroscopy. And that is assuming it is found as a liquid. If you find it as a powder than you may need liquid chromatography and triple quadrupole mass spectrometry. It would take the university where I did my post graduate work weeks if not months to identify these things. How can the government confirm anything?
Here is a great article about the difficulty of actually knowing what a fentanyl analog is: