BNR stands for Buy and Replace. It's a treasury. Shops will buy from other shops to have their shop in the BNR. Buy into a BNR and one of your items will replace the one you purchased. BNS, Buy and Stay, you buy in and are added to a list for the next round.
I don't know much about SEO, still reading up on it. But you're right on what it stands for...that much I do know! LOL