I believe as soon as the Walking Ballista hits the graveyard, Scrap Trawler determines X = 0. Since X from the 2nd Ballista in graveyard is also zero, neither moves.
No. A Ballista's converted mana cost on the battlefield is 0, and so is the converted mana cost of a Ballista card in the graveyard. Barring copy shenanigans, the only place where the converted mana cost of that card can be other than 0 is when it is a spell on the stack, which Scrap Trawler doesn't care about. 0 is not lesser than 0, it's equal. A Ballista dying will never allow you to return an artifact card with Trawler, as no card has CMC lower than 0.
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
The Walking Ballista on the battlefield has mana value 0 (since the X in its mana cost is 0 while it's not on the stack [C.R. 202.3e]) (C.R. 202.3). When it's put into a graveyard, no artifact cards can thereby be targeted with Scrap Trawler's ability, since they can't have "lesser mana value" than 0 (C.R. 107.1b).
EDIT (Aug. 8, 2018): One rule was renumbered with Commander 2018.
EDIT (Nov. 24, 2018): Correctness edit.
EDIT (Dec. 24, 2021): Update terminology.
Warning issued for unsure answer. -MadMage
EDIT (Aug. 8, 2018): One rule was renumbered with Commander 2018.
EDIT (Nov. 24, 2018): Correctness edit.
EDIT (Dec. 24, 2021): Update terminology.