My understanding from past readings (not an expert!!!) is that fighter jets, like high end battle radars and to some extent many other weapons, are "stickier" (my word) than ordinary goods because their geopolitical implications - but also issues of supply and training.
Specifically, because they do require complex supplies and maintenance to function, and they function better when operators and maintenance staff are well trained, "buying" such an item tends to lock you into the selling country's geopolitical sphere if you want to keep using the weapon. Something like, if you want to buck the wishes of the seller, you have a few months or a year to use the weapon before you run low on parts. As a practical matter, you end up partly under the thumb of the seller.
You can't just buy fighters like lollipops. (So far, at least. I think.)
I think the "these fighters are really complicated the Ukrainian couldn't maintain them", is basically an excuse to not send them fighters because some in the government fear this would be an escalation.
A couple of examples. In the mid-1970s, the Shah of Iran bought 80 F14 (the plane of Top Gun fame), at the time it was the most sophisticated fighter in the world. In 1979, the Iranian revolution happened, the US embassy was stormed hostage were taken. The US imposed unprecedented, crippling sanction on Iran. At the top of the list was any type of arm sales, especially spare parts for the F-14, they have remained embargoed ever since. Between 1980 and 1988 Iran and Iraq fought war similar in size and scope to the current war in Ukraine.
According to Wiki, 23 year later of the original 79 F14 sent to Iran 40 remain flying and in service with the Iranian. The US Navy retired the F14 more than a decade ago.
Simlar to the large number of 1950's cars that still remain in service in Cuba, despite being embargoed for over 50 years.
Likewise, I am pretty sure that Russia cut off part supplies for MIGs and other Russian equipment after the 2014 invasion, and in fact it is always been hard for former Warsaw pact countries to get parts for their Russian equipment since the 2000s.
How can Iran keep their planes flying for 40+ years without access to spare parts? Air forces, and airlines, have always made use of hanger queens, planes that have lots of maintenance issues, that eventually become sources of spare parts. It is also the reason that, despite Boeing, and Airbus cutting off services to their planes in Russia, Aeroflot is still flying within Russia and limited number of international flights. I predict will still be doing so years in the future.
So while you are right the seller has lots of power in these deals in war time or hardship people are innovative in finding solutions.