I've bought a lot of used stuff over time.
One thing to note is that pannier styles (and even rack styles) are slowly evolving over time.
In particular, the triangle shaped panniers are less popular now.
There are also some mighty good roll-top and otherwise waterproof panniers available. Ortlieb, and a few other manufacturers.
For racks, look for something with decent rear support.
Another thing that is growing is "bike packing", which tends to have bags tied more securely to the bike, and less reliance on panniers.