According to Wade Lippman :
All appliances require a means to disconnect them within sight of the
appliance, or a means to disconnect them and then lock them out.
A plug will obviously meet that requirement; but presumably your breaker
will accept a "lock-off" device.
So, my understanding is that the sales person is wrong in general; however,
your town may have more restrictive requirements than normal, or they might
not accept the "lock-off" device. I would call the town if you are
concerned.

Here (Canada) whenever you replaced a stove or dryer you _had_ to install a
4-wire outlet, full stop. This dates back at least 30 years.

I hate to throw in another complication, but current code requires both a
neutral and a ground, and you don't have them. Ordinarily you would not
need to update your wiring to meet current code if you were just attaching a
new stove, but if you change something on the circuit (such as installing an
outlet where there wasn't one before) the finished product must meet current
code. Again, your town may or may not enforce it this rigorously.

As I understand it, while Canada insists on going to 4 wire on ALL installations
(or renovations), US code has grandfathered 3 wire to the point where just
putting an outlet on a 3-wire circuit won't require an upgrade to 4-wire. Ask
an inspector to be sure. This will probably vary widely from place to place.

[This is despite the fact that in the US, they permit you to treat conduit as
a ground, hence a conduited 3-wire circuit is trivially upgradeable to 4 wire,
but in Canada, you cannot (legally) use conduit as a grounding conductor.]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.