NFC for example has been demonstrated over significant distances (~10m), albeit with a pretty sensitive (read: sizable) antenna.
I'm more curious how you or the customer thinks an electronic device is usable at all in such an area. Knowingly, anyway. Perhaps that's the point, and it is literal spy equipment; which, AFAIK, is not at all illegal to make and sell, by itself, but don't be shocked if you have suits asking for your sales history at some point, I guess? (It's safe to assume they have identifying information; there are legal processes to discover your forum email, IP addresses, VPN accounts, etc. Maybe not if you've managed to do all that via Tor or something (does Tor interface meaningfully with the rest of the internet, I don't actually recall?!), but if someone like the NSA took interest, they're able to piece together network logs and deanonymize across that.)
In any case, the crime would most likely be the customer's; you would only be culpable I think if there's no obvious or common lawful use of the device. Though they might still get you involved legally, and then you'd have to prove as much to a judge, or jury, at your expense. Maybe factor that consideration into the design/sale price (which is perfectly reasonable: legal costs, criminal or otherwise, are just another business expense), and perhaps consider business insurance.
Likewise, it would be the customer's burden to prove that, for their purposes, your device is adequate for their task, whatever that may be; legally speaking, your intent isn't to enable unlawful acts (...one would hope), and if your device just happens to radiate detectable signals, that's their problem. One must assume spy agencies know enough about things and stuff to test their equipment in detail, and will select from many possible devices, or perhaps modify them to their own ends. If your device is accidentally undetectable, that's not necessarily your problem... but again, proving that in court might not be something you want to risk.
Or something like that. IANAL, not legal advice, etc. etc. Definitely sounds like something you might want to check with a lawyer first.
Tim