ram.exs

src = %{
 first: %{
   buy: [
     %{cost: 90.394, supply: 2.245},
     %{cost: 90.15, supply: 1.305},
     %{cost: 89.394, supply: 4.3}
   ],
   sell: [%{cost: 100.003, supply: 2.4}, %{cost: 100.013, supply: 2.2}]
 },
 second: %{
   buy: [%{cost: 100.01, supply: 4.0}, %{cost: 91.33, supply: 1.8}],
   sell: [%{cost: 90.15, supply: 3.4}, %{cost: 100.15, supply: 3.8}]
 }
}

src
|> Enum.reduce(%{buy: [], sell: []}, fn {_k, v}, acc ->
  acc
  |> Map.put(:buy, acc.buy ++ v.buy)
  |> Map.put(:sell, acc.sell ++ v.sell)
end)
|> IO.inspect()