You know.
You don't need it for YOUR collection, but it's a great deal and/or you know someone else "out there" will want it. Which means this item can eventually be used to get something you really DO need!
Ever do that?
Give me some examples.
Here's a 'three-levels-deep' version. I bought several lots of Genesis cartridges because Genny is getting hot; I stayed away from common lots whenever possible, and accumulated a pretty good pile of quality, complete Genesis games - all of which I already had in my collection. Traded the bulk of those away for a boxed Vectrex Imager. I needed the box but already had an Imager. Now I'm in the early stages of a deal with the Imager that could get me something I've been dying to have in my collection. TIC. TAC. TOE.