<?php

$mysqli = new mysqli('localhost', 'sales_user', '12345678', 'sales_thing');

if (($com = $mysqli->prepare('select id from salesmen'))
    && $com->execute()
    && $result = $com->get_result())
        while ($row = $result->fetch_row())
            generate_random_rows($row[0]);
else echo $mysqli->error;
echo 'done!';

function generate_random_rows($salesManId) {
    global $mysqli;

    $count = rand(0, 300);
    for ($i = 0; $i < $count; $i++) {
        if ($com = $mysqli->prepare('insert into sales(sold_at, salesman_id, product_id, qty) values(date_add(now(), interval round(rand() * 90 * 24 * 60 * 60) second), ?, (select id from products order by rand() limit 1), round(rand() * 2) + 1)')) {
            $com->bind_param('i', $salesManId);
            $com->execute();
        }
    }
}