Posters are a strange, resilient category. Film one-sheets, gig posters, propaganda archives, Swiss design reprints, dorm room decoration, museum shops. The market spans a few dollars to five figures and none of it has gone away.
Posters.org is the plural noun for all of it. It suits a marketplace, a print-on-demand storefront, a collector reference site, or an archive. The archive case is the interesting one. Poster collections are held by libraries, political movements, and design schools, and a .org address is what those institutions already use. A digitised poster archive with this name would look authoritative from the first pageview.
For a retailer, the value is different but just as real. You’re buying the search term and the shelf position in a buyer’s head at the same time. No brand-building budget replaces owning the word for what you sell.
Clean, plural, one word, no spelling problem in any market.
The name is listed for sale. Offers welcome.