I've included several eBay searches in order for you to see what is available and for you to get an idea of the price ranges of collectible vintage Halloween Postcards.
I've bought most of my post cards from eBay and I haven't been disappointed yet. Just be sure to read all of the fine print and check for comments on the condition of the card you are purchasing. Mint is best, poor is worst and anything in between should be carefully explained.
Art Print of Vintage Postcard

Art Print">Halloween, Witch and Cat on Broom
Art Print
18 in. x 12.5 in.
Buy at AllPosters.com
Framed Mounted
I wish I had the magic answer for you here but determining the price of a vintage Halloween postcard is basically the same as determining the price of any collectible. Your postcard is only worth what someone else is willing to pay for it.
Of course the rarer your card is, and the conditon of your post card play a huge roll in determining it's value, if no one wants it, you won't be able to sell it for much.
To get a general idea of what your particular vintage post card is worth you will need to do a bit of studying. Get a price guide. Study the fluctuating prices at ebay for a while, not just a day or two, but for at least a month. Visit vintage Halloween postcard dealers online.
After you've done all of that, compare the condition of your card to the others that you have seen and then you should have a pretty good idea of what your particular postcard is worth.
Art Print of Vintage Postcard

Art Print">Merry Halloween, Cat in Jack O'Lantern
Art Print
12.5 in. x 18 in.
Buy at AllPosters.com
Framed Mounted
Related Articles: