don't give up!
when you post an auction through Auctiva my guess is that it goes into a queue to be posted to E-BAY. sometimes it takes a few minutes. if you re-post it you will end up with a duplicate.
i'm also guessing that when you get that 'not posted in time' error, it means that the listing did not post because either E-BAY or Auctiva was too busy at the time to complete the API call.
at that point it should be safe to either re-post or re-schedule the listing without creating a duplicate.