Yeah, it's pretty easy. You can subscribe to an RSS feed of a stock ticker, and parse that for your updates. Storing that in a database, you then just need a front end so that people can log in and view their portfolio and make trades.
Depending on how in-depth you want to go with this, you then need to have That's just for a 'play' system. If you were doing realtime, you'd want to pay for a dedicated line going into your database to get the stock values that were available, which would come with an API so that you could purchase as well as receive stock values.
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