this one depends on which starter item you're using. I would say that more often than not, the special effect(s) of your upgraded starter item is something you don't want to pass up. However, there are certain ones that you can sell and pick up another item for better/different stats or effects. Usually, this occurs with starter items with an effect that has a long cooldown. For example, The Alternate Timeline (one of the upgrade options from Sands of Time), which revives you upon taking fatal damage, has a 6-minute cooldown before it can proc again. So, if you've got the gold to spare, you can sell this item after its effect procs and get another item instead of it.