* Buy the cheap hardware at wholesale prices
* Strip the surveillance tech and/or re-flash the firmware
* Sell the "dumb" product for the same price as the "smart" one
In this case, the "smart" vendor would likely lose money if they're selling below cost. So the only difficulty would be a legal one... I'm not sure if it's legal to buy a product, modify it, and then re-sell it.
Doing this as a sale, for electronics, you'd probably fall afoul of certification problems (e.g. whatever boxes Underwriters Lab had ticked wouldn't apply to the modified version). There would be branding issues. And regardless of whether it turned out to be legal, you'd 100% draw a lawsuit from the original manufacturer.
If it were to work out at all, this would probably have to be done in a way that (legally at least) the customer is not buying the device modified from you. They would have to purchase the device first, then bring it to you to modify, for which you charge a fee. Even then you'd probably still get sued.
Not a lawyer, though, so this is just speculation based on following tech over the years.
It's both practically unfeasable for the average consumer, and a legal risk for whoever manages to do it.