Is it okay not to leave feedback?

Looking for some opinions about not leaving feedback for an ebay purchase. Here's the situation. I bought a group of around 100 cards described as ex/mt to NM. It was about a $18 purchase with $5 shipping, $23 total. I receive the cards quickly, well packaged but only 1 card of the 100 was in the condition described, the other 99 were good to ex at best with a number of them with creases. The seller had 100% feedback and had been on ebay for a number of years. The pictures weren't great so I was relying mostly on the sellers rating and description. They have a return policy so I can return them but it isn't worth the time for me to package, go to the post office, and pay to return a $18 item. I also don't want to leave negative feedback because they do have a return policy. I have chosen to not leave feedback, and just keep the cards without bothering to contact the seller. my concern is am I doing another potential buyer a disservice as this seller mostly sells lower priced items and maybe he consistently over grades and is it okay to just not leave feedback and move on. Thoughts?
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Having said that, there is a big difference between cards that PSA will grade as NM and what the average person grades as NM.
When I buy a NM card on ebay, I'm elated if it actually turns comes back from PSA as NM.
Mike