The demo is okay; you can complete it in about 20 minutes. There are a few small issues, mainly related to navigating between different options. Here's a list of what I found:

-The market value is always hidden on the main screen, so you have to constantly stop playing and go to the market to check the current value and how much time you have left before it updates. In fact, sometimes I'd have a value of 2.4 (which is an awesome price), and to maximize my earnings, I'd check how much time I had left, click as much as I could, and count in my head how many seconds I had left before selling. This could easily be solved with a panel in the top-left corner of the screen showing the current market value, with a small progress bar underneath representing how much time is left before the price updates. The panel could even have a small button that takes you directly to the market.

-There is no direct way to go from the market to the upgrades panel. Most of the time, you'll want to sell everything and then go to the upgrades panel, which is basically the main gameplay loop: Click->Sell->Upgrade. However, the way you currently have to do this is a bit awkward. First, you have to click the options button, then the market button, then Sell All, then close the panel, then click the options button again, then click the upgrades button, and finally select the upgrade you want to purchase. This wouldn't be a problem with the rebirth system, since you won't be constantly clicking the rebirth button, but for something as common as the main gameplay loop, you'd want the process to be as smooth as possible.

-Is there a reason for having the Sell 10% and Sell 50% options? I mean, the only real currency is money, and the buttons are only there to be sold. There is no reason you'd want to keep half of your buttons for later, so I think those two buttons are pointless for now. Sell All is the only option that really makes sense at the moment, unless the dev adds some new mechanic that actually uses the buttons for something else.

-There's a bug at the start of the game where you click "Play" and then "Yes", but the game just lingers there for a few seconds before the transition into the game starts. You can even click "Play" again during that time.

-Regarding the content, I think it's nice as a metagame, like a critique of how we play games, or as an exercise. However, I can't really see this becoming a full commercial game that people would pay money for. Maybe it could work as a mobile game with ads, but that's about it.

So, those are my thoughts. I hope they help!