If you go to antique stores and select old post cards...you can find these for like .10 - .25 cents...maybe a dollar...you want vintage postcards that have like a building with a "coke" sign on the wall...or old name brand advertising from the 50s and such...these will sell for like 10 -35 dollars on ebay...check it out...
I think most of the people selling post cards, stamps or ebooks are just there to increase their feedback ratings. Once they have the sufficient feedbacks under their belt, they start selling some expensive stuffs like electronics or jewelery.
feedback manipulation is against ebay policies. if you sell cheap items to get feedback and then suddenly jump to expensive items they will limit your account.
I have sold batches of "vintage" postcards on eBay and usually made only a nickel or two on the transaction. I suppose someone who knew what to look for to try to flip might do well, but it would require far more expertise than just looking in garage sales for old postcards.
That can be a great idea in selling post cards, but do you made the post cards or you get it from someone?
It's under 'collectables'. Just had a look at the first page on Ebay UK showing old postcards - hardly any bids made on them - maybe it's better in other countries.