Obviously you can race with other people. As for trading it was on GT5 until PD and Sony realises people would abuse even the PSN accounts for such. PD nerfed the trading greatly with subsequent patches on GT5 until it was cut on GT6 altogether. Theres is no online market where you can sold your car to other people. Though there are online dealership but it only contains zero miles cars that are discounted and it's only on GT5.