As I understand it, the license needs to be obtained by the manufacturer, not necessarily the reseller. Sisvel is the company that developed decoding of mp3 codec, and they have the patent on the process. I have purchased mp3's from China, and the mfr. usually offers it with or without Sisvel. It costs them less than $1 per unit for Sisvel, but as we know, people will try to take short cuts whenever possible.
The mfr. can get you the paperwork documenting the license for the products, but many times, the reseller has no idea if the license was actually obtained if they are buying it through secondary channels. I do not know of anyway to verify if the Sisvel license was applied to any particular unit after the fact. It is something that should be supplied when selling down the chain for proof to the ultimate buyer.