I don't really sell anything on the market, but if it prevents anything from being listed and available it's a bad thing I think. It's one thing to pay a fee for a whole machine, but a lot of the small parts, mods, etc will never be worth it. Also there is a lot of fan stuff on this site that I don't people are making much of a profit on, so they probably just won't list them.
I decided to donate when I was able to successfully find multiple odd parts I would never find elsewhere. Not via the market, but by talking to people on the forums. I think if you get a regular benefit from the site for any reason you should donate or support the site in whatever way you can.