If you've got loads of obsolete tanks, of which some are kinda sorta working, others are mostly useful only as spares and your mission is to turn that into money but without harming national security, selling say half a dozen of them to a movie props company, makes lot of sense. Deactivate the main weapon, remove auxiliary weapons, strip out any sensitive materials from the interior, and it still looks like a tank on screen, but it's not a real threat because it's deactivated. They can use it to shoot war movies, as background for politics movies, &c.

Serious adversaries don't need obsolete tanks. If you're Russia, a working 1970s tank is of no value to you. Russia have perfectly good tanks newer than that, and their crews were already trained on them. However you don't want some minor power or non-government entity to buy these and re-arm them somewhere that can't defend itself, you don't want your obsolete tanks to show up in a guerilla war in South America, so you can guarantee that with "Not for export" type paperwork.