There are several sites that are available to sellers now on-line.
If the cards are vintage you can sell them on Artfire.com. I think they still have a beginner offer where you can list less than 10 items for free. If you want to list more than that you pay a monthly fee to the site.
Etsy.com will also do vintage. You pay .20 cents per entry and then a small percentage of the final sale price as commission.
IOffer.com is another site that I've shopped at but have not sold at.
When I have something to sell I do a search on one of the items and check out the sale sites that come up. That way you can also get an idea of what the market price for your items are right now. That helps when figuring out your price, don't forget you should add in the percentage that you will probably need to pay to PayPal for the transaction.
Hope that helps.