Respectfully request what to look for when shopping for a used carb

Rebuild what ever you buy and run it.

You need to know what you're doing before start.
There are guidelines.
If you're going to have a 4 Corner idle then the idle screws are going to want to be 1/2 to 3/4 turn out on all four idle mixture screws. If you're going to run a vacuum secondary Holley it's going to want to be about 1 and 1/2 turns out on the idle screws. If you're going to run a Edelbrock or AFB style carburetor they're going to want to be one and a half to two turns out.
Based on ootb specs.
Theres no need for testing if it's got new gaskets in it, no load break in... you're good.
What are you doing?