Looks like your quoting gets a bit funky here within the VALUES expression:
IF ($_POST["action"]=="add2"){
$result = mysql_query("INSERT INTO inventory (Brand, Stock, Model, VIN, Description, Retail, Cost)
VALUES ("$_POST[Brand] .", "$_POST[stock]","$_POST[model]", "$_POST[VIN]", "$_POST[description]", "$_POST[retail]", "$_POST[cost]")");
}
What you probably want is something like:
IF ($_POST["action"]=="add2"){
$result = mysql_query("INSERT INTO inventory (Brand, Stock, Model, VIN, Description, Retail, Cost)
VALUES ('{$_POST['Brand']}', '{$_POST['stock']}', '{$_POST['model']}', '{$_POST['VIN']}', '{$_POST['description']}', '{$_POST['retail']}', '{$_POST['cost']}')");
}