Can a poshmark business count as a passion project?

I would consider this a form of self-employment and put it under work experience. It's no different than working in a resale clothing boutique except that you are the owner/proprietor and your store is a virtual closet on Poshmark's selling platform. You pay yourself out the profits and re-invest some of the proceeds by acquiring more items or making more items. So you are running a small business for yourself.

For it to be a passion project or something you do to fundraising for 3rd party, first of all, I wouldn't comingle your personal account with the funds you collect that are intended for the non-profit company. Also, I would look into alternative platforms other than Poshmark because 99% of Poshers are not fundraising. I'm not sure if Etsy is any better but definitely not Mercari or eBay. Perhaps you can do some research and figure out what the best venue is for fund-raising for the special non-profile you want to donate.

A more impressive EC would be to fundraise by selling the things you make and start your own non-profit and donate directly to projects that improve the environment where you live. Maybe you can rent a tent at a local farmers' market or Saturday market and sell your things directly to the public. Or go in on it with 3 to 4 friends that have different handmade things to sell as well, so you can all be working toward a common goal of fundraising.

Have fun with it and enjoy your summer.