I am sorry, I wrote it down a bit clumsy (and wrong). 

If the `delimiter` is set (`Some("/")` by default on our buckets), the `NextMarker` element is returned. This is because you can theoretically get a different `NextMarker` than the last item in the previously returned list.

So I think we should follow the documentation:
- Check for `IsTruncated`
- Check for `NextMarker` (or `delimiter.isDefined` on the bucket)
- If `NextMarker` is present, use that, otherwise use `listResponse.last.name`

The related part in the documentation:

> When response is truncated (the IsTruncated element value in the response is true), you can use the key name in this field as marker in the subsequent request to get next set of objects. Amazon S3 lists objects in alphabetical order. 
>> **Note**
>
>> This element is returned only if you have delimiter request parameter specified. If response does not include the NextMaker and it is truncated, you can use the value of the last Key in the response as the marker in the subsequent request to get the next set of object keys.