http://futures.tradingcharts.com/marketquotes/JM.html#footer...
You could do it on the market and make money (I know of a couple companies that do this). But, the technology is applicable to other areas, as well.
RTOs (and ISOs, which are similar) are non-profit organizations that ensure transmission grid reliability and fairness by operating markets in which generators and distribution companies bid or buy energy [1]. The market mechanisms have been designed to also incorporates network balancing and congestion control. RTOs/ISOs are the benevolent, omniscient regional gods in charge of coordinating dispatch for the purposes of ensuring transmission grid reliability. They facilitate sales, they don't sell directly.
Transmission companies, on the other hand, are for-profit organizations that build and maintain transmission lines. Their profits are constrained by regulation to be a certain percent return on equity [2], and they need to get permission from the RTO/ISO that monitors their region before making any operational changes to their transmission lines.
One more difference is in their scale. There are only ten RTOs/ISOs in North America, and they tend to cover very large regions of the continent. There are many transmission companies, and they tend to cover smaller regions.
For example, where I live in Southeast Michigan, the RTO/ISO is the Midcontinent Independent System Operator (MISO), and the transmission company is ITC [3], a spin-off from DTE Energy after deregulation forced DTE to pick two from their generation, transmission, and distribution capabilities (they kept generation and distribution, because post-deregulation that's where the money is). However, MISO's entire coverage area includes 52 transmission companies [4].
Basically, under deregulated energy markets transmission is treated as a public utility and the transmission grid is treated as common infrastructure. The grid is operated by non-profits and private companies who are required to be financially independent from generation and distribution interests. That's why the notion that distribution companies buy energy from transmission companies gives the wrong impression of how the system works, though I can see where the confusion could come from.
[1] https://en.wikipedia.org/wiki/Regional_transmission_organiza...
[2] http://archive.jsonline.com/business/lower-profit-recommende...
[3] https://en.wikipedia.org/wiki/ITC_Transmission
[4] https://www.misoenergy.org/StakeholderCenter/Members/Pages/M...
Interestingly, renewables still comprise a tiny fraction of generation. I'm not sure if this is because of regulatory requirements, lack of reliable storage, or because of cost (nuclear and fossils are pretty cheap and reliable producers).
Generally, in the US, distribution companies (i.e. electric utilities) purchase energy from the transmission companies. Many don't generate electricity themselves. Actually, most distribution companies are co-ops. There are still regulation via FERC, and they stay active because of high-impact capacity transmission failures in the past.
But, it's all cool stuff. And, for those interested in machine learning, you can download a lot (or all) of the data and make a lot of money (I used to work in a pseudo-quant like capacity in that space).
http://www.pjm.com/markets-and-operations.aspx
https://learn.pjm.com/three-priorities/buying-and-selling-en...
http://pjm.com/markets-and-operations/energy/real-time/histo...