I have used RentAcoder with some success (and some failure)
I think the Keys to success are to do the following
1) Have a solid Spec (With Screen Mockups)
2) Ask them to read the spec before bidding
3) Question them on the their proposed design, ask them how they will implement certain features , then ask them Why that design (This is particularly easy because you are developers as well)
4) Understand that you will need to do the QA, and compensate them for change control
5) Reputation matters a lot.
You will know right away who knows what they are doing, and who you can work with. Dismiss any answers of “1 great system coming up! No problems at all!”
The bids will be insanely low, so you may want to hedge your bet and hire multiple teams just in case one fails.
Also know that once the engagement is over, there is a good chance you will never be abul to contact them again. So you now own the code.
Lastly no matter what you do, there is a high chance of Failure... Just accept this a fact of hiring cheap labor.