Stocks can't really be used; the only thing you can do with them outside of trading is using them to view the status of that faction.
To buy or sell stocks, there are a few places this can be done:
1. The Galactic Market: www.swc-gm.com (basically SWC's version of eBay)
2. The RPG Centre -> Commerce or Auction House, depending on if you want to sell or auction it off.
3. The #swc-traders IRC channel, if you want someone to purchase it real-time.
Always remember to use a good middleman when trading with a person you don't trust. Two resources which list most good traders and thieves are:
1. Enigma's Traders Reputation List - http://members.chello.nl/~o.koek/traderslist.htm
2. Traders Protection Agency (TPA) - http://pub238.ezboard.com/btradersprotectionagency