I'm not sure how you could do that, Maggie. (If you're talking about product pages and not forum discussions.)
I always add info about series and/or sets to my description, and make sure to show the pieces next to each other in the actual gallery(ies). At least, that way, the buyer knows where to find the other piece(s).
One of my train sets (2 pieces) has sold here, multiple times -- the pieces have never sold independently. So, I guess some buyers pay attention! :-)
