If you have any real photo postcards I would list those first. They are worth more than most other cards. You will want to make sure they are not manufactured. Check the back for publisher info. If there is a publisher, such as LL Cook, they are not worth as much. Look through the Real Photo Postcard RPPC category on ebay for the highest priced sold cards and you will quickly be able to see the difference in manufactured real photos and real photos. With that said, some of the manufactued photos still do fairly well, especially street views from the 40s and 50s. Real photos of people don't sell well, unless it is a really rare view of an outlaw, or if is a funny studio prop card (such as a person posed on a funny plane - or with their heads poking through cutout holes at an amusement park).
Advertising postcards sell well.
Santa Claus in any color other than red sells well if it is a good quality card (especially German Santas).
Search for completed postcards on ebay and see if you have anything that sold for over $10; being sure to note if there were also others of the same card that did not sell.
Don't list anything that others have tried to sell and didn't. If you do, you will be wasting your time. Maybe save those for last and let them sit as Good Til Cancelled listings so you don't have to worry about them.
If you have anything that "looks" really rare to you, see if you can find it online. If you can't find it, list it.
Good Luck!
