This has more to do with low barrier to entry in any online platform. For any job there could be zillions of bids, going down spiral. All claiming they can do the job. It creates a sort of situation where people don't realise that their task is to find needle in a haystack

As someone who has worked on oDesk and made good money. You also need to do client filtering.

1. Check their total expense

2. Number of people hired, on hire

3. How much are they paying them

4. Feedback given

All this information is available.