Man, I should've kept all the NOS moldings that the previous owners of my '84 El Camino gave me when I bought it! They had the car quicky painted and they removed all the studs to mount the side trim. So I never put them on, and then I gave them to the guy I sold the car to back in '02...
Are they in good shape? They better be nice for that kind of money.
I'd ask for lots of pictures, or if you can, inspect them yourself for damage before you buy them. I hate seeing dinged up and damaged moldings on a nice car... But I'm kinda picky since I restore the stuff...
There are a few different styles of clips used to retain the moldings.
Did your car come with these style moldings from the factory, or are you going to be adding these onto a plain jane model?
You best bet is to get your hands on a GM/Fisher Body assembly manual.
It will have detailed pictures of the trim clips, as well as how they mount, and part numbers. Granted, the numbers won't mean much, but they at least provide the opportunity to cross-reference them with something current.