I'm looking to buy a used backpack in the near future. What are the key things I should look out for/avoid when inspecting used paks?
Some thoughts about qualities of a (big) backpack for multi day hikes:
I think the most important is to check if the backpack actually fits your back. You should be able to adjust it so the belt can run comfortably around your waist, while shoulder straps can be easily adjusted to your shoulders - there should be no gap between shoulders and the straps, nor they should be too tight. A backpack that does not fit is definitely not worth buying.
As for the sings of wear and tear, first check if all the zippers work (and still have a nice sound :)) then check the material bag is made from for scratches etc. Check it from both sides, as the inner side might be scuffed and the bag then might not be as waterproof as you'd expect. Lastly, I'd check all the seams and all plastic buckles - especialy on the major parts - each of the straps etc.
The key points to look for are wear at the joins between webbing and buckles, or between webbing and the fabric of the pack itself, as these will typically go first.
Then look for rips, tears or areas worn thin in the fabric of the pack - on all sides and underneath. The top is probably less of an issue.
After that, check the frame for splits, dents or bends - depending on where these are they may not be a problem, but still worth looking at as they indicate something happened to the pack.
Then I would check zips and extra webbing straps - I worry less about these as I can always replace or repair them, but they can be annoying if one is missing or broken.