Neutrals can sell to anyone. Sweden and Switzerland did massive sales to Germany during the war especially of military items that Germany couldn't source elsewhere and rare raw materials for the German war industry. For that matter Russia made WWII possible for Germany by allowing the Germans to conduct banned military training and research inside the USSR under the veil of the communist country AND providing Germany with the raw material, food and fuel necessary to invade western Europe. Not to mention conspiring with Germany to invade Poland and divvy up Eastern Europe between Germany and the USSR.
Inconvenient truths that many chose to ignore.
The US was Neutral until 1941, when Germany declared war against the US. The US was even neutral in WWI, until after an almost-war the US gave an ultimatum to Germany to stop attacking neutral shipping. Germany's response was to attempt to convince Mexico to attack the US, an act of war that resulted in the US joining against them.
And it always amazes me that more people are not aware of the Kama Tank School, the Lipetsk Fighter-Pilot School, and the Tomka Gas Test Site. All three joint German-Soviet programs conducted in the Soviet Union in order to help both sides develop better weapons and tactics. All done to circumvent the demilitarization provisions placed on Germany after WWI.
But yes, neutral nations can in fact sell to anybody. That is a key part of "Neutrality" in the first place. That was a key factor in Spain remaining Neutral in WWII. While nominally a satellite of Germany, they were also completely reliant on imports from the US (mostly fuel, oil, and coal) so it was either remain neutral or their lights would go out and their people would starve. And there was no way that Germany could have made up those shortfalls if Spain did join them in the war.
And there was actually communication between Spain and Germany informing them of when shipments of supplies from the US were arriving and warnings to not attack them. And Germany let them pass almost completely unmolested because the last thing they wanted was to piss off Spain and have them join in the war against them. Just like Japan allowed supplies sent to the Soviets by the US to pass unmolested, because they did not want the Soviets to join in the fight against them.
Nations at war violate neutrality at great risk, because doing so can have impacts far beyond the expected. Even causing other neutral nations impose embargos or even joining in against them for fear that they might be next. Look no farther than the number of nations that had been neutral since the end of WWII suddenly joining NATO from fear of the recent Russian expansionism.
