Knowing before they tell you.
WHAT TO MONITOR
Whether each site responds Whether it responds correctly, not just with something SSL certificate expiry Domain expiry
WHY
A client who tells you their site is down has already lost confidence.
Telling them you are already fixing it is an entirely different conversation.
WHAT IT COSTS
Basic uptime monitoring is inexpensive or free for a modest number of sites.
For a reseller, it is one of the highest-value small expenses available.
WHAT TO CHECK BEYOND UPTIME
That the page contains expected content. A site returning a database error still returns a response, and a naive monitor calls that up.
CERTIFICATE EXPIRY
AutoSSL usually renews automatically. When it fails, the first sign is a browser warning on a client site.
Monitoring expiry catches it first.
DOMAIN EXPIRY
Track every client domain's renewal date. A lapsed domain is a serious and entirely preventable failure.
ALERTS
To somewhere you actually see them, and not only to email on the domain being monitored.