I was reading something on a totally different subject that made me think of these watches. Basically, they're pricing an item that if it worked would be a) of interest to many of us and many healthcare systems and b) probably very durable, you'd expect years from it.
Given that the cost to the user for 2 weeks of CGM is >£50, you might expect that people or healthcare systems might pay a comparable amount for a non-invasive watch device that accurately monitored BG. Let's say you discounted it a little so that the watch purchase cost only the equivalent of £40 a fortnight (a saving against CGM of £5/week). That comes to just over £1000 for 52 weeks. And if the watch lasts 5 years, you'd be looking at a price around £5,000. Obviously that gives you loads of room to reduce the price further (once you've sold to all the people who'd pay £5,000) while still making huge profit. I expect the CGM manufacturers have been down this road already.
So why are these things being advertised so cheaply? If they worked, they'd be worth an awful lot more, thousands of pounds more. My conclusion is that they are so cheap because they're junk, and the fact they're so cheap proves they're junk. And when they prove to be junk, no-one will chase them for being scammed out of £70 the way they'd be chased if someone had spent £5,000.