The real nice thing about Costco is that when I see something I like, I know that it is of decent quality at a decent price. This saves me from having to do research on my own.
You know what was nice? They have my phone number and called me when they announced a peanut butter recall. Well it wasn't that nice to find out I'd been eating potentially poison peanut butter, but it's better than other retailers.
Costco will shut you down if you start playing the 'buy it -return it' game too much. They monitor that. Stand around the front desk a bit and watch the used up crap people return.
We have returned food. Sometimes it has been stuff which we didn't like the taste of or produce which has gone bad very quickly. We don't do it often but it happens. Never had any issues in returning it.
You can 100% return food. I've returned fruit before that was spoiled and some Kirkland brand k-cups which were the worst thing I've ever bought from there.