If the postcards have some meaning for you, I would suggest that you make some form of collage and frame it. Perhaps arranged by theme. Cityscapes, nature, streetlife, buildings, country, age. I had some small value bills from different countries I've been to over the years, and did that with them. I steam-ironed them to remove wrinkles and folds. It became a very nice "painting" and it wasn't even very expensive, despite being made from money. People seem to check it out much more than my other paintings and etchings...
There are DIY frames with glass available in craft stores.
There’s only one thing you can do with a collection of post cards. Lay them out on a table, and take a lot of photos of them. List them on Ebay as an auction, and let the collectors or resellers price them. It is impossible for you to try and price them yourself individually.