Why shouldn’t i buy random used panels and bolt them onto my roof with net metering?

Any reason why this would be a bad idea? I see used panels come up like this all the time. Ameren MO offers a pretty decent net metering plan I wouldn’t be trying to get my bill to zero but maybe 15 250 watt panels, on paper this works out to offset me bill nicely even if it doesn’t cover everything. It seems every installation guide I see is about getting your bill to zero but I feel like there’s a diminishing return on the work involved and the space available for me. Anyone done something similar?