Hi Angela,
 
I've not heard of anyone here trying to sell individual greeting cards there.  Here is the problem:
 
Ebay insert fee is 35 cents (a cost even if it doesn't sell).
If you sell a card, the Ebay commission is 25 cents.
 
That's 60 cents of cost against our GCU commission of 56 cents.
 
Perhaps buying GCU cards in Qty and making up sets to sell on Ebay might be workable, but I am sceptical.  
 
 
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UPDATE
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I did find one Card Store on Ebay.  http://stores.ebay.com/Made-4U-By-Faith
(She apparently makes her own cards somehow)
 
She charges $3.32/card and $2.17 S&H, so with some mark-up, even individual GCU cards could sell profitably perhaps.  We'd have to take in the order via ebay/paypal then turn around and place an order on GCU and have them send the card directly to the end customer.  Hmmm.....

I know several who sell their cafe press items on Ebay- maybe that is what you are seeing- or perhaps they print their own?

We have a ebay store  at
http://stores.ebay.com/valxart  
and fees can add up quickly even if there are no sales as it often is when your starting up a store.  
 
Its cheaper to list items in a ebay store than on auction, but it still cost a insertion fee of .05 for a single title listing per month. We don't mind commissions on sales, but fixed monthly cost add up quickly. We also list items with ebay givingworks which donates  20 to 40% of sales to non-profit orgs but the same listing fees apply even if no one buys. We have done this only a short while and its far to early to say it works well.  
 
Listing fees for 80 items currently in our store and  store monthly fees amounts to $25 a month.  If we added the best 1000 card designs our cost would rise to $75 even if there are no sales. This encourages you to list fewer items.  
Ebay fees are more acceptable on high ticket items.
 
All this shows why i love GCU
 
At GCU We are free to list lots of designs and let buyers decide for themselves .
We find it hard to predict what people like. We have images we were not wild about and are delighted to discover people connect with them and buy them.
If I had to pay monthly for each listed item I would only list the items i liked and  would miss out on discovering what customers  like.