It is a good idea, and has been used for quite a few years, however you may find an even better idea to formulate a monthly or
by-monthly e-newsletter for your customers, or even a combination of both.
There are two forms of marketing, pro-active and re-active.
A business card is considered re-active as it can only bring you return business if your customer is actively searching for a product he thinks you may well have. Your card reacts to their requirements at the time by leading them back to you to fill that requirement.
Impulse purchasing makes up a massive amount of the average persons spending habits, and a proactive e-newsletter lets you get your message in front of their eyes constantly, and gives you the opportunity to create an impulse purchase.
You can show your customer's new stock, offer specials......even competitions for purchasing within a time certain frame, whatever! All of these techniques are proven impulse sale techniques, and can increase your bottom line with minimum effort.
Customers are also more inclined to forward an email to a friend if they think they may find it useful, where as a business card won't be given unless a friend asks!
A business card won't put your name in front of their eyes as much as you need to be to really create return business, if that is your aim.
Of course if you have stagnant stock and nothing new to offer, then you're wasting your time even with business cards, unless you are selling every day consumables.
The aim of marketing is to get your name and your products in front of as many people as possible, and keep it there as much as possible!
Choose the more aggressive pro-active marketing techniques over re-active techniques, and you give yourself the best possible chance of increasing your profits.
Don't think for a minute that because you don't consider yourself a business that you can't use business type marketing, it's what makes private sellers successful.
Plenty of email options, constant contact is quite a good one, but plenty of other. I would always suggest using HTML, not simply text. You are trying ton create desire and a really high impact platform has a much better chance of delivering that message effectively.