On most shared hosting plans there's a limit to the number of emails the server will allow any one domain to send each hour. How involved is it to build something that allows the user to specify the number of emails the system can send each hour? And if the email count goes beyond that limit the emails are queued for delivery during the next hour?
Its pretty involved because you not only need some php knowledge but sendmail knowledge as well. My personal opinion is that if you are serious about running a real classified biz then you need to stop skimping and get a real hosting account. The next best thing would be to implement one of the many open source mailing list scripts to handle you mailing needs.
