The incentive to sell not-MDMA as MDMA is self-evident; the drug is popular all times of the year (high demand) but the ability to make it requires an in-depth knowledge of organic synthesis and access to resources (low supply), some of which has been made illegal in recent years (even lower supply). Therefore, being criminals by definition, vendors would like nothing more than to sell GHB, DXM, speed, etc. and have you and everyone else believe it's MDMA (or at least invalidate your claims it's not) as there is a countless amount of drugs that are significantly cheaper to obtain and much easier to obtain, giving them a stake in the MDMA honeypot.
Testing kits have become the de-facto standard in ensuring the powder you received is in fact MDMA. If the color goes straight to black, the powder is undoubtedly verified to be MDMA and the case is closed. Simply put, testing kits do NOT have the ability to test your powder for MDMA content. The ONLY thing testing kits are good for is telling you whether you surely don't have MDMA (negative). For example, if the Marquis reagent turns red, that powder is 100% something else. If it tests positive, however, it could still be anything.
As the popularity of testing kits has grown exponentially, vendors have caught on. With a lot of money at stake, you can believe they do their very best to ensure any disputes to their legitimacy be disqualified. Thus testing kits while at first serving a noble purpose have become misleading and a source of false confidence.
How does one spoof a testing kit? MDMA and a Marquis reagent, for instance, should go straight to purple or black. However, this coloration overwhelms all other colorations, so a bit of MDMA mixed with a lot of not-MDMA would produce results that pass the powder as good. The relative amounts of the drugs used and the identity of the drugs would all affect the outcome. MDMA is not even necessary; one can find the right combination of various drugs (or not even drugs at all) to elicit the same much-desired color profile. The extent to which it will mimic the reaction of MDMA (or indeed be identical) needs to be scientifically tested. However, given the drive to do this (big money) and the relatively trivial task at hand (make a solution turn black) you can rest assured this is done.
Do the darknet markets no longer contain a source of MDMA? No. Someone somewhere on some market surely is selling real MDMA. Do more vendors than we think sell not-MDMA? Yes. The extent to which this is done is hard to quantify as the new age of the darknet and the recent immortilization of testing kits have made it impossible to gauge what is legitimate and what is not.
This knowledge has been known as far back as 2001: http://www.bluelight.org/vb/threads/27066-can-dealers-do-anything-to-fool-the-testing-kit-results. If this was being reported 15 years ago, it's hard to imagine why every single vendor would not ensure his powder passed a testing kit, MDMA or not.
The Catholic Church hid a bunch of pedophiles in its system, leader of nations all over the world have recently been shown to be in a widespread tax evasion scheme, the NSA spies on all of you, and vendors are engineering their powder to spoof an MDMA testing kit and destroy any dissenting voices. In an illegal business like this, you can be sure of that.
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