Is the Listing Service the only way to get IPv4 addresses?

No. The RIPE NCC allocates the IPv4 address space it holds according to section 5.6 of  “IPv4 Address Allocation and Assignment Policies for the RIPE NCC Service Region”, which states that RIPE NCC members can request a one time /22 allocation (1,024 IPv4 addresses) if they already have an IPv6 allocation. No new IPv4 Provider Independent (PI) space will be assigned.

Any transfer of address space via the Listing Service must comply with section 5.5 of Address Allocation and Assignment Policies for the RIPE NCC Service Region.