I suggest two related features:
- On users' exchange pages, we have an option to show only coins that we don't have (by type). It would be nice to have a button "show only coins from countries from which I don't have any coins".
- On coins' description pages we have a link "Members from this site want to exchange this coin". Imagine similar link "Members from this site want to exchange coins from this country" (on pages of countries).