Separate registration and login for different roles

Creating two separate registration for different roles is easy:

//create a hidden field for role
add_action('register_form','add_hidden_role_field');

function add_hidden_role_field(){
    if (isset($_GET['role'])){
        echo '<input id="user_email" type="hidden" tabindex="20" size="25" value="'.$_GET['role'].'" name="role"/>';
    }
}

add_action('user_register', 'update_role');

//save the the role
function update_role($user_id, $password="", $meta=array()) {
   if (isset($_POST['role'])){
       $userdata = array();
       $userdata['ID'] = $user_id;
       $userdata['role'] = $_POST['role'];

       //only allow if user role is my_role to avoid a few new admins to the site
       if (($userdata['role'] == "seller") or ($userdata['role'] == "buyer")){
          wp_update_user($userdata);
       }
   }
}

and now you can link each role with “its own” registration form:

seller: http://example.com/wp-login.php?action=register&role=seller
buyer: http://example.com/wp-login.php?action=register&role=buyer

but as Milo commented:

“if someone registers as a buyer,
there’s no way they can log in as
anything but buyer with their
credentials”

which means that they would have to use a different email to register the other role.

Update

this is an update with an example to show how you can use the same fore but with different fields for each role.

So you just need to change the functions a bit:

//create a hidden field for role and extra fields needed
add_action('register_form','add_hidden_role_field');

function add_hidden_role_field(){
    if (isset($_GET['role'])){
        $user_type = $_GET['role'];
        echo '<input id="user_email" type="hidden" tabindex="20" size="25" value="'.$_GET['role'].'" name="role"/>';
    }
    if (isset($user_type) && $user_type == "seller"){
        //add extra seller fields here eg:
        ?>
        business name:
        <input id="user_email" type="text" tabindex="20" size="25" value="" name="business_name"/>

        business address:
        <input id="user_email" type="text" tabindex="20" size="25" value="" name="business_address"/>
        <?php
    }
    if (isset($user_type) && $user_type == "buyer"){
        //add extra buyer fields here eg:
        ?>
        buyer name:
        <input id="user_email" type="text" tabindex="20" size="25" value="" name="buyer_name"/>
        <?php
    }
}

this way only the fields needed by the specific role are shown.

Next is if you want to have some kind of validation to these extra fields you can use register_post hook for example:

add_action('register_post','my_user_fields_validation',10,3);

function my_user_fields_validation($login, $email, $errors) {
    global $firstname, $lastname;
    //get the role to check
    if (isset($_POST['role'])){
        $user_type = $_POST['role'];
    }
    //check the fields according to the role
    if (isset($user_type) && $user_type == "seller"){
    //check sellers fields
        if ($_POST['business_name'] == '') {
            $errors->add('empty_business_name', "<strong>ERROR</strong>: Please Enter in a Business name");
        }
        if ($_POST['business_address'] == '') {
            $errors->add('empty_business_address', "<strong>ERROR</strong>: Please Enter in Business address");
        } 
    }
    if (isset($user_type) && $user_type == "buyer"){
        //check buyers fields
        if ($_POST['buyer_name'] == '') {
            $errors->add('empty_buyer_name', "<strong>ERROR</strong>: Please Enter in a Buyer name");
        }
    }
}

then if every thing is fine just save the fields in the user meta based on the role

add_action('user_register', 'update_role');

//save the role
function update_role($user_id, $password="", $meta=array()) {
   if (isset($_POST['role'])){
       $userdata = array();
       $userdata['ID'] = $user_id;
       $userdata['role'] = $_POST['role'];
        $user_type = $_POST['role'];
       //only allow if user role is my_role to avoid a few new admins to the site
        if (($userdata['role'] == "seller") or ($userdata['role'] == "buyer")){
            wp_update_user($userdata);
        }
        if (isset($user_type) && $user_type == "seller"){
            //save sellers fields
            update_user_meta($user_id, 'business_name', $_POST['business_name']);
            update_user_meta($user_id, 'business_address', $_POST['business_address']);
        }
        if (isset($user_type) && $user_type == "buyer"){
            //save sellers fields
            update_user_meta($user_id, 'buyer_name', $_POST['buyer_name']);
        }
   }
}