That so-called customer (your website buyer) is a scammer himself and the owner of the exchange website you mentioned. So, first of all, they bought a website from you, which is a scam because you didn't received your money. 2nd, they scammed your own money which you had deposited to their platform in order to get your money. So, he basically scammed you twice.

Never accept login credentials as a payment. I assume he asked you to create an account in this exchange and he said he sent funds to your account, right? Actually he did not sent any funds or did not deposited anything. He just added a balance from the admin panel and you got scammed. Stay away from these scammers.