I'm pretty sure that BNR and BNS are interchangable (Buy 'N Replace, Buy 'N Stay). It means that if you buy something from that particular treasury, then the seller you bought from gets kicked out and you get featured in their place. If someone then buys from you, you get kicked out and that other person takes your place. The idea is to buy your way into these treasuries.
And don't feel dumb! We were all wondering what the heck these abbreviations meant once, too! :D