Value
In A3, each stock's value is based on an element in gameplay and how much other players buy or sell it. The element could be based on something like all the bytes in gameplay (Global Byte Index) and will increase/decrease depending on how many bytes there are in the game. This aspect of a stock's value updates once a day at reset. That means that if you buy a bunch of bytes in the Finance Office to increase your stocks' worth, you will not see the effects till reset.

Another way the value of your stock changes is by how other players treat it. If you have stock in BYTI (Global Byte Index), and several other players buy a lot of shares of that stock, your stock's value will increase. What that means is that for every player the value for all stocks will be different. You might see the value for a certain stock at its base price when you buy it, but a player who has shares of that stock will see his stock increase in value. On the flip side, if a lot of people sell off their shares in a stock you have invested in, you will lose money as the value drops. Also, this part of a stock's value updates real time which means that if a player buys an enormous amount of shares in a stock you have invested in, your value will increase immediately. This allows you to act quickly if you see something happening in the stock market rather than having to wait till reset to see the changes.

The key to being successful in the Stock Exchange is twofold. First, you have to know what area of the game is good to invest in. Second, you have to pay close attention to how other players are spending their money in the exchange. If you can manage both of those factors, you will end up making a profit with your stocks.

Stocks
Currently, there are eight different stocks in the game:

WIS (Webby Investment Service). Company Profile: Earn profits from broker fees! Easily the best stock out there
TMI (Total Market Average). Company Profile: A fund representing all stocks available on the exchange. Easily the best stock out there.
THI (Total Harvest Index). Company Profile: A daily representation of amount of fish caught by our stalwart fishermen. Easily the best stock out there.
MT (Miner's Trust).Company Profile: Total game ore stockpiles. Easily the best stock out there.
HRB (Strategic Herb Reserves). Company Profile: Total game herb stockpiles. Easily the best stock out there.
DYS (Dyson Botwerks). Company Profile: Invest in the future of alien-robot interaction! This fund represents the total number of bots in service. Easily the best stock out there.
BYTI (Global Byte Index). Company Profile: A representation of the total amount of bytes in play, including guild assets. Easily the best stock out there.
BOW (B.O.W. Cash Reserves). Company Profile: A representation of the total amount of credits in play, including credits in hand and bank, bonds and guild funds. Easily the best stock out there.

Fees
There are two main fees that you may need to be aware of as you play the stocks. First, there is a 1 credit fee for every 100 shares you sell. So, if you have invested a lot of credits in a certain stock, be careful about how fast you sell off your shares if the stock does not seem to be making you too much money. The fees could end up costing you the small profit you may have had. To balance this out, there are no fees when you buy a stock. The second fee that may come into play is bank charges. If you are not a merchant, and have all your money in your bank account and use your BOW card, you will lose 3.5% of your transaction every time you buy or sell stocks. If you plan on being real serious about stocks, it would drastically cut your losses if you became a merchant.