It's melatonin; melatonin is so cheap that you actually wouldn't save much, if any, money by sending your customers fakes. And the effect is clear enough that they'd quickly call you on fakes.

And they may look shady simply because they're not competently run. To give an example, I've been running an ad from a modafinil seller, and as part of the process, I've gotten some data from them - and they're easily costing themselves half their sales due to basic glaring UI issues in their checkout process. It's not that they're scammers: I know they're selling real modafinil from India and are trying to improve. They just suck at it.