I was freelancing for many years - mostly blue chip multi-nationals. I worked via a company for tax purposes and legal protection etc etc - they took 5% of my money, but also (because of their size and deals with the tax man) got me a lot of tax breaks (such as claiming food allowances and car milage back from tax) - which more than made up the 5%.
However, I worked alongside a smallish company for almost ten years in one major retailer. All their workers were good contractors - none were paid a perminant wage or got any employee type protection or bonuses (profit share etc). What they did get was agreed contractual rates with the umbrella company (that is what I would call them) and they didn't have to hunt around for work. The unmbrella company did the deal with the contractors after doing a deal to supply contractors en-bloc to the businesses. That way, the deal between the umbrella company and the retailer is not relevent to the contractor and umbrella's contract.
The umbrella company works out the margin, takes the risk, pays the contractor (with escape clauses in case the project closes early) and takes in the profit. In some way at looking at this, it is no real difference between any shop buying from a supplier and selling at a profit to the public - the supplier could make more selling direct (and some do of course), but that is harder work, piece-meal, riskier and requires premises etc - so each rubs the other's back.
The difference here would be that you would be doing the bidding on ODesk (which is good as the rep builds continually against your group name and not half a hundred individuals) and once awarded you choose the contract and sub contract the work.