I've done that twice. Once with a man who was thinking about getting a Pacu, and once with a man who wanted a different variety of fish (that weren't at all compatable). The man/pacu deal was simple. I saw him looking at the Pacus and saying how much he liked 'em/wanted one, and I told him that they can get to 2+ ft. long. He went "WOW" then thanked me and walked away. . . . The other dude I was talking about was a different story. He was stubborn and really didn't want to listen. He wanted 3 Angels, a Figure-8 Puffer, a (common) Pleco, and a Neon Tetra (yes, "a" as in single). After a while I talked him into only getting one Angel. It kinda feels good knowing at least a couple fish won't be going to un-knowing fish owners, and more than likely too-small tanks. 
