Just a rod and waders in a glass case? Was the display helping promote some other products or did they have a stock of the rod and waders for sale, too?
If it was just a display, it likely was designed by a contractor who bought the items to create an attractive 'draw' to get folks looking at other fishing-related products that are actually for sale. Very common in retail -- for instance, a lot of the stuff you see in big department store window displays isn't actually sold by the store, it's merely added by the designer because it looks good, works with the marketing concept, and helps draw people into the story/department/area to buy what the store DOES have for sale.
Then again, if there was a stack of Simms waders and Sage rods nearby then Costco has some explaining to do -- or rather, the wholesale distribution centers who work with those brands have some explaining to do since most speciality outdoor brands have rules about where their products can be sold.
Google " Missoula Costco , Simms Waders" and you will see they have them on their warehouse list of items for sale as well as a Sage rod.
This is sad as I just took a last trip down to my local fly shop for his going out of business sale.
Both companies sell through Cabela's, so Costco really isn't that big of a leap. It's just another example of consolidation of the retail channel in the flyfishing industry: Small retailers go out of business, and many equipment manufacturers decide that they must sell through big box and Internet stores to reach customers.