the First Sale Doctrine or something like it generally applies, and the purchaser gets to do all sorts of innocent things
The First Sale Doctrine is a US legal precedent which does not apply in the UK. Also: The first sale doctrine generally does not apply to Unpublished work -- suppose I wrote a book, never published, but lost a copy of the book, Or the company I had do the printing made an extra without my knowledge and was lost in shipping. In theory you could find my lost book, but No first sale exists, since the copyright owner did not authorize the original first sale - there are no first sale rights that can flow to the holder of the physical copy.
Thus even in the US; this precedent would not save you, because the console was property of Nintendo, and never sold with their authorization. The copyright owner never authorized a sale. In short, there is no first sale, because Nintendo did not sell it, and neither did they ever grant anyone else the right to sell the copy.
The Merchant's mere intent to traffic in it by reselling it further can thus constitute not just Copyright infringement with commercial gain, but Patent infringement as well. (Nintendo's hardware designs covered by multiple patents)
It is a remarkable stretch to claim that copyright extends to turning something on. Copyright doesn't deal with that sort of thing at all.
Under UK Copyright law: Powering a device up and loading software into RAM counts as making a new copy of the software, and it is a violation of the law If you are not properly licensed. You also have no fair use rights if you possess software which was not first sold with the authorization of the copyright owner. Since Nintendo will never have intended for the Dev Kit to be released to the public -- the copyright work will be unpublished, And the copy has never been sold with the authorization of the copyright owner -- therefore, It is not possible for the Merchant to convey a legal copy or license to their buyer, since the unit was never legally sold with distribution authorization of the copyright owner to begin with.
Powering the console on loads some kind of software into memory. Even if Nintendo has implemented security protocols which prevent you fully booting the dev kit without authenticating your entitlement.
Copyright covers the distribution or trafficking in works as well. The copyright holder has the right to control distribution of -- in what is in this case unpublished work.