It really sounds like this was mdma.... I mean unless it was actual moist beach sand in a bag? I mean how hard are guys going to try to sell fake mdma when they can make even more money off mephedrone now... Why not buy a sample before any big purchases so you don't screw yourrself... I friend of mine got a shit ton of molly about 10 years ago. We would take the equivalent of 2 match heads and put it in a clear capsules... Rolled hard.... twas incredible...
I can assure you this was not MDMA in the least. I did get lucky later and run into some beautiful shards of mdma that tested great both with the marquis and ingestion. But the stuff that turned yellow was being sold as mdma but was definitely either mephedrone or methylone judging by the effects.