I ran into the same problem awhile back and it took me awhile to navigate the site to get the information I needed.
Go to HOME (1st item, 1st line directly underneath QUILTING BOARD at the top center of the page.
Scroll down the list of topics and click on ALL TOPICS.
Scroll down and click on BUY/SELL/TRADE
Just above all the listings is a column header 'RULES' - you should find the reason why in here ... and I am suspecting it's because you're not designated a "regular" [poster]. It tells you what you need to do to become a "regular" - which is either you post 20 responses to items within 2 weeks' time ("regular" status) or make a purchase of a book ("community benefactor"). I myself bought BOSTON BLOCKS for our quilt guild and they have turned out some amazing charity quilts while buzzing through mountains of donated scraps.
While it may seem to be a pain, it protects the vast majority of good folks from not-so-nice predators who are new (not "known" as a presence on the chatboard) and wanting to take advantage of others. Someone new/unknown could come in, offer something for sale, get the money, not deliver the goods and bolt. Same thing for someone new/unknown asking for a donation of fabric, unused machines, [whatever]. It's a good rule.