Okay, got it.
I think letting the listing owners manage their own tags for this purpose would become unwieldy and unmanageable very quickly. 🙁
Perhaps a better approach would be to extend the functionality of the related post system to be able to use the listing’s Title Field to make connections? If it’s not a huge amount of work I would be willing to commission you (or a developer you work with and trust) to customize this.
Would this functionality be a valuable addition to the core? Not sure if other users would find this useful or not but it provides a necessary competitive edge for our application.