I'm from Jakarta, frankly this not simple like this:
'beli' = to buy
'membeli' = is buying
Beli is a short term of membeli, let me give u an example:
- Saya beli baju
- Saya membeli baju
Both sentences are correct! What's the different?
- Saya beli baju ... when you speak
- Saya membeli baju ... when you write
No one in Indonesia say "Saya membeli baju di pasar", instead we say .. "Saya beli baju di pasar". But when on a school we write "Saya membeli baju di pasar". But currently most Indonesia use "Saya beli ..." rahter than " Saya memberli ...".