Update: This is from Ger the pinoy linguist ! I will just take a few hours to research what all these words mean, and I should have my answer. I'm glad there was an answer.
Here's the answer to your question:
Pwede bang bumili ng roomba
you have ba + na because "pwede" takes an entire clause as a complement
na introduces subordinate clauses
Saan ka ba bumili ng roomba
There's no subordinate clause there
The declarative version of that sentence is something like Bumili ka ng roomba sa SM
"Saan", the question pronoun, replaces the noun phrase "sa SM"
And then, like question words often do, it gets moved to the front of the sentence
*Saan bumili ka ng roomba
But clitics have to go in the second position in the sentence, so you get the right order
Saan ka bumili ng roomba
You want to add ba in, it goes after the pronoun clitics:
Saan ka ba bumili ng roomba
There's no subordinate clause here, so no need for "na"