Think about it. There is no way anyone is holding anything since the ban, 13 years ago? I even know people who tried and stashed 25kg barrels of it in the country side. His supply didn't last more than a few months, back then everyone was fiending it.
From what you've said, I would interpret it this way; the vendor means by pre ban and post ban, is one batch hes got is actually 4-MMC, and one is something else.
Seeing the wedinos test from the vendor, is good they are testing, but nearly all people selling powder. Cocaine etc now are having wedinos tests. That doesn't stop them sending out the same product cut down or something else entirely. To be honest the best relations i have made were from getting a vendors stuff tested, then showing him i know what he sent was not as advertised. After that everytime was the best of the best.