Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a deeper, technical meaning?
Non-portablility isn't about sales... it's a technical term.
Non-portable IP addresses belong to a certain organization, and an end-user of the non-portable IP address is
not permitted to discouraged from announcing those IP addresses to another organization without the owing organization SWIPing the addresses to the end-user.
The concept of portability comes from the ARIN Number Resource Policy Manual 2014.1, Section on IPv4 (emphasis mine)...
4.1. General Principles
Provider independent (*portable*) addresses issued directly from ARIN
or other Regional Registries are not guaranteed to be globally routable.
The reason the RIRs don't guarantee that portable addresses are globally routable is because the RIRs can't influence route filtering policies of providers.
Why some blocks are non-portable
Addresses are either allocated or assigned. If an address block is assigned to the ISP, then that ISP cannot sub-assign the addresses. Quoting ARIN Number Resource Policy Manual 2014.1, Section on 2.5
2.5. Allocate and Assign
A distinction is made between address allocation and address assignment,
i.e., ISPs are "allocated" address space as described herein, while end-
users are "assigned" address space.
> Allocate - To allocate means to distribute address space to IRs for
the purpose of subsequent distribution by them.
> Assign - To assign means to delegate address space to an ISP or
end-user, for specific use within the Internet infrastructure
they operate. Assignments must only be made for specific purposes
documented by specific organizations and are not to be sub-assigned to
Registries operate in accordance with RFC 7020, which replaced RFC 2050. Arguably, much of the structure around these restrictions comes from RFC 7020 Section 2, Goals:
2) Hierarchical Allocation: Given current routing technology, the
distribution of IP addresses in a hierarchical manner increases
the likelihood of continued scaling of the Internet's routing
system. As such, it is currently a goal to allocate IP addresses
in such a way that permits aggregation of these addresses into a
minimum number of routing announcements.
You asked about "the block or a subnet"; however, ARIN deals with allocations of individual IP addresses, not subnets or blocks of addresses.