Not technically correct, shops, like any seller, are required to be satisfied the buyer has a defence according to the vcra, obviously for shops ukara is the easiest, but not the only "paper" defence, some legitimate, some slightly shady.
When it comes to private sales, some sellers are adamant they'll only accept ukara, but ultimately it's about convincing a seller that you're a legitimate player, something I've been able to do even though I haven't had ukara for more than a decade.
That is of course on the rare occasion a sellers asks for proof of a defence, most just want to see the colour of your money 😏