In Indonesian, present tense and present continues tense are the same.
So, naturally, we don't distinguish the difference between 'Tina buys clothes' and 'Tina is buying clothes'. Both are 'Tini membeli pakaian' for us.
But if you really want to stress on the ongoing action that Tini is doing now, you can add the word 'sedang'.
'Tini sedang membeli pakaian'.
In that sense,
Membeli = buy
Sedang membeli = is buying
But remember, the word 'sedang' is optional. Without using it, your sentence will be understood.