In regards to CA specifically, it doesnt matter if they are pre ban or not.
If a person owned them in CA before the ban they were allowed to keep them.
Importing them, whether pre-ban or not, is illegal in CA.
However, "parts kits" are allowed to be imported into CA. If you disassembled them, you could sell them into CA as "parts kits". Perfectly legal and allowed.
What they buyer does with the parts kits is their issue.
Essentially, they are allowed to rebuild their existing mags... every single piece can be replaced. But they cant end up with more mags than what they started with.