I haven't found one specifically on here, but the google machine usually works well ;)
For apples, basically look for ones that don't have soft spots. Green beans should be firm and, well, green. Eggplant, I have no idea. For things like peppers and tomatoes, find ones that don't have soft spots.
Avocados are a bit more difficult. You want one that's slightly soft all over (perfectly ripe). If you aren't going to use one for a few days after you buy it, get one that's firm to the touch and it will ripen on the counter.
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