Is it permissible to sell a marriage website, and what is the ruling concerning the money previously acquired from it, given the continuing problem of users' lack of seriousness?

If the website serves those who wish to marry and observes the Sharia regulations, then continue to provide the service, seeking reward. It is permissible to sell it, provided that the purchasing party does not use it in ways that are impermissible according to Sharia. You should have monitored the website due to the prevalence of its use in forbidden matters. The money earned is lawful if it is in exchange for a permissible communication service or advertisements that contain no prohibitions.