The Russkies never captured a Buk from Ukraine. NATO saw them 2 weeks prior to the shootdown with Buks on the Russian side of the border, conducting training. The Russkies then claimed they captured one (Ukraine denies) and then sent them across the border, along the same route everything coming from Russia drives through. Typically, whenever the Russkies send something over the border, they claim they captured it first, to maintain plausible deniability. But there is no evidence for it except for claims from Russian state controlled media, with no other independent sources.
As for the whole "duely elected government was over thrown." The Interim government all along was the Ukrainian Rada elected in 2012. Only Yanukovych was "overthrown," and this was done by him fleeing the country to a foreign enemy, not by anyone forcing him out. This is like Obama fleeing from Washington DC to Iran. Or Bibi fleeing to Hamas, after turning into a liberal. Yanukovych could have fled anywhere in Ukraine, even to the East, but chose Russia. The Ukrainian SBU says they think it was Russia that forced him to flee to their country.
Suspicion for the apparent shooting down of a Malaysia Airlines flight immediately fell on pro-Russian separatists in eastern Ukraine who two weeks ago boasted of capturing powerful surface-to-air missiles.
Anton Gerashenko, a senior aide to Ukraine’s interior minister, said the aircraft carrying 295 passengers was brought down with a missile fired from a Buk anti-aircraft missile battery.
Fighters from the Donetsk People’s Republic (DPR) overran a Ukrainian army garrison on June 29 and claimed to have captured at least one such weapon. “ http://www.telegraph.co.uk/news/worldnews/europe/ukraine/10974821/Ukrainian-separatists-suspected-of-bringing-down-Malaysia-Airlines-flight-on-Russian-border.html
Now where is your source? Nato?