* Traton may also co-list in Stockholm - sources (Adds quotes, details, banks): FRANKFURT (Reuters) - Volkswagen is aiming to list its trucks unit Traton in April and expects to sell shares worth 5 billion to 6 billion euros ($5.7 billion to $6.9 billion), two people close to the matter said. Plans for a stock market listing, potentially Germany’s biggest new share offering in 2019, have not been finalised and depend on market conditions, these people said.