using a SUM and ISBLANK includes filtered data

Hey guys, 

I have a model with two tables 1 fact (Transactions) and 1 dimension (Seller).  There is a one to many relationship from seller to transactions.  I have a report level filter on the seller so that I can view transaction data for a single seller. 

I first created a measure TotalTransacting = SUM(Transacting).  However, I wanted to have default values of "0" where there was no data to display to avoid gaps in things like date ranges.  So I tried to modify the query in two ways both are giving me issues.

TotalTransacting = SUM(Transacting) + 0

TotalTransacting = IF(ISBLANK(SUM(Transacting)),0,SUM(Transacting))

The problem I am seeing is that when i use these measures the filter on sellerKey seems to get ignored and I am getting data for all sellers.

Here is a screenshot to help visualize:

You can see that when i drag the sellerkey from the seller table into a table visual it is filtered properly to the right seller.  But the seller in the transaction table is not getting filtered.  As soon as I change the measure back to a simple SUM the sellers are filtered out but again this is not ideal for my situation as i want the default to be a zero and not blank while still respecting any report filters I have in place.  So in the case of the visual above i would expect 1 row with the seller I'm filtered to in the report with a "0" value for my measure.  How can I accomplish this?