How to post in "for sale" section?

Hello, I've tried to read the FAQs, but I don't see what's required to be able to post in the "for sale" sections. There's an item for sale that I REALLY want to buy, but I cannot respond to the post.

Sorry if I'm missing something obvious. Thank you!

Thanks, I'm happy to donate to become a premium member, but I don't see where to do that. I'm not seeing it under my account settings or anything.