[OT] In the past, at the university and before getting a job in the industry, I used to freelance on all of these portals, doing random programming work and language translation (tiny jobs from $15 to $500). Nowadays it's true that even for us in Central-Eastern Europe, the prices undercut by even lower income countries are an obstacle. It isn't rentable to take such a job anymore. I haven't taken a job there for many years mainly because of the prices. I guess the only field left, for now, is human language translation, where there really aren't even a few individuals in all those BILLIONS who'd speak any of our many languages But for IT or even ART, ... It's hard to find any quality in the swarms of the absolutely low-cost jobs, you'd have to 'sample' many times until you find a really good freelancer (for example to help you with coding, art, testing, yet alone marketing). That's a risk. And at any time they might stop freelancing. [/OT]
It is possible, though, to get work done via freelancing portals. I wouldn't say likely, but definitely possible.
As for games marketing, I understand that (indie) studios haven't even started requesting marketing services through these portals yet? I'd try other channels - go to local game conferences and off try to offer your services to those who present their soon to be finished games, for example.