Sales associates receive product training in the product the store carries. You shouldn't expect them to know about other brands. Most of them are just trying to make a living. They are not SA's fo life. It's not a calling.
As for buyers... there is an odd phenomenon. ime, a majority of buyers do not ever go into a competitors store, or do so very infrequently. Which means that they do not necessarily know about product in the same pricepoint that they buy at, which they do not carry. Also, there are a billion brands out there. Buyers have a few weeks every season (and less, if they are ower/proprietors of small businesses) in which to see all of these brands and write their orders.
As an aside, this means that they may attend a few trade shows, but they will often also rely on a small number of showrooms, and a few multi-brand showrooms have enormous power over the buying public. For example, chances are that if, to go outside of menswear for a moment, a store carries Burning Torch, they will also likely carry Velvet, Inhabit, Graham and Spencer, maybe A Common Thread, and... in all likelihood, Gary Graham, if they have a rich enough clientele, because not only do all of these brands share a similar vision of airy, feminine styles with a bit of an artisanal conceit, are represented by Findings in NYC. And you think that Seven New York is so outre and daring? Nah, they just buy a lot from People's Revolution, where you can pick up Jeremy Scott, And I, and Bless, all at the same time. You like upscale hipster fashion? Get yourself directly to Denise Williamson to pick up Spurr, Chimala, Duckie Brown, Linda Farrow, and Oliver Spencer, and then onto The News, to pick up Common Projects, Band of Outsiders, and Woolrich Woolen Mills. Sound familiar? I think that the News has got the lockdown on the downtown crowd. (I think that the News probably owes Styleforum some commission.)
You wanna start a Eurotrash store? There are a few select showrooms for you as well.