Heres what I've read about purchasing a Dutch Oven: First, how many people are you cooking for?
Cast Iron or aluminium- I prefer cast iron because it cooks evenly, holds heat well and I dont have to worry about burning things.
Look over the oven, make sure the lid sits on top without any wobbling, check for openings that could mean the oven or lid is warped.
Check that the walls and bottom are the same thickness, this makes for even cooking.
Inspect the whole thing for defects inside & out.
If considering a camp oven, pick it up to see how the bail moves, putting it on the floor & lifting it like its coming out of the fire is a good test.