I had a question a few days ago about about used vs unused postcards, which do we (and others) prefer? It's a very good question, and goes to the heart of why people collect postcards. From a purely selling to others point of view, it really helps if to have an idea of what your potential customers want, or why they collect.
People collect both used & unused cards, extensively. I almost never ask anyone why they wanted a particular card, and on those all too few occasions when they bid up the price of a card to a point I never dreamed of asking, I never ask why. Some people volunteer information about why they buy certain cards, and I've researched around a bit & come to my own conclusions, so perhaps I have the beginnings of an understanding about this.
For those that volunteer information, the most common reason is the card means something to them personally. Just recently we sold a card that was a view of a lake in Washington state. The person who bought it said her in-laws lived on the shores of that lake and would like the picture. Many postcards have sold for similar reasons, and the people who buy them for this reason usually don't care if they're used or not.
Some people collect series of cards - we sold some Diefenbach Silhouette cards, Union Oil Cards, Advertising cards and others where there is a limited number of cards in the series. People who collect things like this, are most likely are looking for a pristine version, but they'll buy a used one, or one of lesser quality as kind of a marker until a better one comes along.
Some people collect postmarks - so they'd only collect cards that are used. The postmark itself is important, the city, how legible it is etc. There are many people who look for postmarks from post offices which no longer exist. If I find a postmark from Bearville, Kentucky, for example, I just might hold on to it. That was a little post office in a general store a couple of miles thru a mountain road from my Granny's house. It took a bit of effort to get from Granny's house to the post office, but that's what grandchildren are for. Just give 'em enough money to buy a pepsi once they got there, and they'd be fine. At any rate, it doesn't exist anymore. Whenever I get a card with an interesting postmark on it, such as Dalbo, Minn., I get my hopes up. If it's a post office that no longer exists, I really get my hopes up.
There are hundreds of reasons for collecting postcards. I'm sure there are reasons or methods I've never thought of. In the overal scheme of things, used or unused doesn't seem to matter.