Approximately how many postcards do you have and are any Halloween theme and are there real photo postcards as well?
The value really depends on the subject matter, condition and publisher, but for the most part, they are generally in the $1 - $10 range. The best first method for research would be though Ebay and checking their past auction sales (which goes back on 60 days) and short specific word search of each of your postcards will likely yield matches or subject comparables to help you gauge sale value.
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