Ok, without talking about design.. where because of my trade I would recommend to get a professional design it for you..
Your best bet is to prioritize what would be the best way for your customer to contact you and find your products. What to add is to your discretion, depending on.. if you want to give your phone number to everybody you meet, even at craft shows.. or if instead you prefer to have everything documented then , email is the best way.
I always recommend to my customers to include pictures of their products, specially if they have a handmade uniqueness to them... this is makes it easy to remember for the customer that picks up your cards, where, when did they see you and what your were selling.. it should be memorable, that way it doesnt get thrown away.
In your case, you have vintage products, it wouldnt be a bad idea to see a sample of the things you have collected or sold in your store, so people have an idea of your taste. In this case, a double sided card with help tons!! because your info would be on what side and the pictures on the other...
Im sure lots of people here, will start to recommend the best place to have them printed at, instead of home-made: Vistaprint, Overnight prints, Moo, etc. but regardless of where you print them, the most important part to actually get value out of them is to have them represent you store clearly and effectively, when even if the customer misplaces the card (it happens very often)... they are able to remember your store name and your product: "I can't find the card anywhere but I remember this cute beagle in it and used the name HoundDogs.." that's enough to find you on Etsy :)