Selling postcards can be profitable, but throwing a bunch of postcards in a stack and not taking time hurts one overall. Member a picture is worth a thousand words. It also means more bids if you have a few cards people want. State what's wrong and be upfront. Personally, I don't like one burries the fact items are not returnable.
Do your research.
A lot of ships in the great lakes sunk. Do any of these cards have a postal history connection (i.e. Postage Due)? Then, you may want to treat a so called commonly seen postcard as a stamp collectible.
Don't rely soley on eBay to find inventory for resale. For example I bought a Cape Elizabeth Fire Station postcard for $2 and sold it for $22 on eBay. There's also a lot of dealers getting rid of excess inventory of cards which at least I keep seeing or are of low interest. There's still stuff out there, but it takes time!
Is it wise to start/quit an eBay store?
I have mixed feelings. eBay keeps raising fees as to the stores, but at least they do market on your behalf and give sales reports. Only eBay is ever mentioned on TV. Stampwants, Bidville are not.
Charging cheaper postage (This is How to do it).
Starting auctions at a penny then charging a few dollars for shipping is wrong to me. Eventually one will lose customers. You can save on shipping by getting card stock and manila envelopes at Sams Club if having access to it.
I'm a buyer, not a seller. By chance, I purchased a card addressed to a family member I was researching. Because I'm interested in genealogy, I now look for cards where the backs are shown or described. Prior to my lucky find, I was only interested in the images on the basis of era and locale.
Large scans are great, but a written description would work just as well. For a buyer, it's not too practical to bombard sellers with queries about who the card is addressed to (or to discover possibly some interesting feature in the message), when this could be taken care of in the item listing.