As a Postcrosser in an 'Exotic' location I get a lot of requests for private swaps. People trying to complete collections from around the world seems to be way more common than you would think.
I don't generally do private swaps but every once in a while I'll get really compelling requests and I can't help but agree to send a postcard. I've gotten ever type of request from travelers making a project for their children to teach them about the world, to terminally ill people trying to make their birthdays special. This site has a way of connecting people who love to collect information about the world even if they can't travel themselves.
One private request I get regularly is for "Greetings From..." cards. For a while I didn't understand what it was about. In the beginning I thought they just wanted a card saying "Greetings From Barbados." However, one of my friends enlightened me that there is a Greetings From Series on postallove.com of different countries, which they are trying to collect. What postcrossers try to do is get cards sent to them from their respective countries. The cards are lovely and some of the locations are fairly random so it looks like it can get fairly difficult.
I'm getting more and more interested. I think I'm going to send myself cards from the countries that I've visited instead of just asking people to send me cards. Although I see the appeal of needing a full collection.
If you want to check out the series go here. Just know it is kind of addictive and you will want to collect them all.