>> Or is it acceptable for the taxman to see a daily or monthly postage cost?
The information you'll be filing with HMRC will be summarised figures for the whole year. Depending on your turnover, this may not even be broken down into categories.
>> I assume postage comes under direct costs of business for a trader who only sells on Ebay?
I think so as the postage would be zero is no sales were made. The seller fee is an overhead (advertising)
>> Can this be totaled as a daily or monthly amount?
Yes, it's not important for the DIY package to have granularity bellow a month, however you should keep what detailed record you have so you can respond to any questions that could come up in the future.