I bought a second hand Mid 2012 MacBook Pro a few months ago. I realized it smelled like a sweaty shirt as soon as I received it. I cleaned it up pretty well on the outside, but it still smelled bad. I read a few comments about this same issue, with apparently no solution, so I kind of assumed I just had to get used to it. But honestly, I couldn’t tolerate it. So when I opened it to replace the HDD for an SSD and upgraded RAM from 4 to 16gb, unfortunately I realized the smell inside was even worst. So I gently cleaned it up with a soft brush. I also removed all the dust from the fan and cleaned the back cover (which was VERY stinky) with a cotton soaked in alcohol gel. Then I left the thing open for a while. It still smelled bad once I closed it, but '''A LOT''' less than before. A few weeks later, I opened it up again and cleaned it once more… this time, it remained open for like an hour.
To help this stinky one a little more, I put incense boxes inside the bag I keep it in, so now it smells exactly as I want it to smell. Seriously, I haven’t felt the bad smell again. Now it really feels like it’s a brand new MacBook, and I’m so happy that I finally got rid of the bad smell that I had to share it. Of course, I always make sure to keep the keyboard, the trackpad and every single part of it very clean.