Just because you think something is rare, doesn't make it so.
Ebay is NOT an indication what things are worth in reality.
I have seen CAAK Il-62 and Tu-154 safetycards go for like US$60+ each on Ebay. Does it matter than I am organising with someone to get a small supply of these for only US$25-30? Are you still going to tell me that CAAK cards are worth US$60+? CAAK cards aren't rare. They are readily available. You just have to know the right people to get them.
The same with Russian stuff. People claim that this is rare, and this is especially rare, just because it is Russian. I am sure than more than one of the 200 million people in the CIS have the same item that you are pushing at rare, and I bet they have net access, and are willing to sell the same item at less than 1/10th of the cost.
Anyway, put your Zambia Airways postcards on Ebay. If you get US$5+ per postcard, good on you, because some moron has just been suckered into getting them. Try listing them at US$1 to start with as well. See how many bids you actually do get on your "rare" cards.
In other words, don't throw the word "rare" about just because you are wanting to be a tight arse with your stuff. Basically, that is what it boils down to.