The first port of call upon buying a defective product would be to return it to the seller, David Eastman's defence team says.

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However, the prosecution case in Mr Eastman's murder trial alleges the accused instead attempted to sell a Ruger 10/22 rifle to a Queanbeyan sports store owner only days before Colin Winchester's assassination.

Defence barrister, George Georgiou SC, on Tuesday told an ACT Supreme Court there were "real problems" with the timeframe and logic of the scenario presented by the Crown.