Umm, in pretty much every country except Russia it is completely legal to violate Russian state secrets, and in pretty much any country other than USA it's legal to violate USA state secrets. The international buyer should be completely in the clear.

Furthermore, it's very likely that the seller also is completly legal - as long as it's outside of Russia but one of ex-USSR republics, their new legal systems generally don't prohibit distributing USSR state secrets, only documents that are classified by their own new authorities.

Can you steal from a state that doesn't even exist at the time?

Yes you can, states that don't exist anymore have other states that inherit both their treasures, secrets, copyrighted works as well as debts. I understand that Russia is that state in general. The issue is complicated in case of ex Soviet Republics presumably.

But I would guess you also can't just storm into Mosfilm or Lenfilm (movie production studios) and start copying all their movies made during the Soviet times and selling them for profit.

Not sure if this applies to the maps in question, but legal rights (and obligations) are handed down to successor country/countries. International treaties, debts and everything. Even Olympic medals I believe.

You are quite right.

Technically, buying them - they were 'acquired' by the mentioned map making companies. TBF said state secrets kinda fell through when the USSR fell apart.