First let me say Im no pro piper. But I have bought me many estates over the years.
Since I do not rebuild pipes myself, just cleaning and mild buffing I look for the following.

1) no major tooth marks on stem, some minor ones are easily fixed, but I do not venture into big bite marks.
2) look all over for cracks in and around stem on pipe and bowl. If the pipe is dirty or heavily caked up its sometimes hard to see. And they won't show themselves until after you clean it.
3) Does the stem fit snug or is it loose?
4) Is it priced low enough for the brand that if after cleaning I find it has issues that Im not gonna be out too much coin?
5) do they have multiple pipes so I can offer a lower price for the lot?

This is what works for me, as an amateur.