NOTICE HOW I NEVER SAID DON'T BUY A RE-POST!
I'd be careful about this one too, but let me explain.
If a larger SoundCloud channel sells re-posts on their page, it's perfectly ok to buy.
That's promotion to me instead of just 'plays'.
Music promotion is different than plays because all it guarantees is to put your music out there. Which is a good thing.
Honestly, who cares if people play your music one time and never listen to it again?
That's not the type of people you're going for.
You're going for the people who will play it repeatedly!