No they can't, and there's precedent for that: food regulations, warranty regulations, refund regulations, regulations on sale to minors, tax regulations etc. - this is in recognition of the fact that without the resources provided by the state and civil society, they wouldn't be able to sell it (or, in some cases, even produce it) in the first place.
> But I could do it with the advantage that their version includes using the machinery of the state to call a restriction of a freedom a freedom.
This is rather ironic, not only because restriction of freedom is what gives us what we regard as higher freedom (almost by definition, e.g. the case of the savage society being less free than ours since there are no laws against murder and theft, thus people cannot pursue their "higher" ends freely) but also because Orwell was himself a socialist![0]
Restrictions on freedom are not merely trade-offs, they are essential to the functioning of society. The point I'm making is that this restriction on the freedom of the seller in this case is based on a conception of freedom in which its opposite (unfreedom) is held within the noun itself. When you speak of the freedom of the seller, you're also speaking of the unfreedom of everyone else.
[0] "Every line of serious work I have written since 1936 has been written, directly or indirectly, against totalitarianism and for democratic socialism, as I understand it." (George Orwell, 1946, "Why I Write")