I got one from them and I also asked them about the warranty. They said they got their order through V-Moda. Your M100 is covered by massdrop for one year. I said on the package it said about 2 years warranty and they replied if it is over one year, you need to contact V-Moda directly.
It still worries me that the SN code are missing on the headphone, box, and little user manual.
I would be very concerned if they removed the serial numbers. That leads me to believe they purchased them from a distributor that removed the numbers to avoid being caught selling them sideways.