[Face9000]

Something like this:

pawn Код:

new carsold[MAX_VEHICLES];

Command:

pawn Код:

CMD:sellvehicle(playerid[])
{
    if(carsold[vehicleid] == 1) return SendClientMessage(playerid,0xFF0000FF,"You have already sold this car.");
    SendClientMessage(playerid,orange,"***Car Sold!***");
    GivePlayerMoney(playerid,5000);
    return 1;
}

But i don't think it's correct,i just gave you an example.You have to get the vehicle id of the driver and put the variable carsold on 1 when you sold the car,and 0 when the car isn't sold.

Remember to reset the variable when a player disconnect or at least put a timer.