You do not notice this, you do not need to enter your private key, but just enter a bitcoin address, and if the private key for that address exists in our database, it will show you otherwise, something to display Does not exist
And from where do you get those private-keys? If that's the case, your website is pretty much useless since there are 2^160 possibilities of Bitcoin addresses and you only have 24,363,716 addresses stored in your database.
Please note that none of the addresses you are looking are not stored in server and all of it in client-side search operations are done.
How is this a client-side if the address is being sent to a page called "search_api.php"?