The Stock Market is used by buying stocks at low prices and selling them at high prices, the prices updates on a interval. There are 5 types of stocks you can trade. You can only buy a maximum amount of stocks at each time which is 500+10*level. As an example at level 1 you can buy 510, 13 you can buy 630 and 50 you can buy 1000.
Known stock prices (please update if you know more)[]
Eastern arms:
10 - 35
Northen Petrol:
100 - 182
Sunrise Medical:
337 - 400
Uptown Brewery:
512 - 615
Venture Co:
1092 - 1458