Use the trading marketplace if you're looking to buy a specific car, looking to sell a car, looking to trade/trade for a specific car, etc.
Posting format example:
Title: For Sale - Stock Bugatti Veyron
Message: I have a stock Bugatti Veyron I'm looking to trade for either an R3 drifting car or 350,000 game credits.
^^^Note, this is just an example.^^^
You can also use abbreviations in the title, such as:
F/S = for sale
F/T = for trade
