The misconception of starting an online store is that many people think that it's easier than starting a physical retail store.
But just because your business is online doesn't mean that it's going to be easy.
You still need to make sure you have enough merchandise in stock, and you have a variety of products you're selling (in order to cater to different needs), and that your products are rotated fairly quickly (no one wants to walk into a store where the same products have been in display there for months, collecting dust... it's the same thing with online stores).
You need to take enticing pictures of your product (photos that'll make people imagine holding your product in their hand or wearing it) so people would be tempted to buy. You can't get away with pictures taken with your phone anymore. If your pictures look unprofessional, customers would assume you're not serious about your online store, and they will feel nervous about ordering anything from you.
So if you're not serious about it, you should think about this. It's going to be a lot of work. It's not going to be a 4-hour-a-week kind of deal here.
Another fact to consider is your product and who it would appeal to.
Generic retail stores will experience a slump. Likewise, generic online stores will also experience a slump. If your product is something customers can get, let's say, at overstock.com, then you are directly competing with overstock.com. And people over there have teams and teams of people marketing the same item you (and you alone) are trying to sell at your little online store. It's not impossible, but it's going to be damn hard.
You need to give a strong, compelling reason for people to come to your online store instead of going over to Overstock. Your product must target specific needs (notice how I didn't say specific people?). We call this 'niche market.'
People can go over to Overstock, let's say, to buy costumes. But if you're offering a pink rabbit costume for men over 7-feet tall, they'll have to come to you to buy that item. The more specific you are, the more easier it'll be.
Conclusion
Before jumping in and starting your own online store, consider all the pros and cons, just as if you would be starting a physical store. It's a lot of work, more work than you might be up for. But if you do it right, it could be something extremely profitable.
As for me, I decided against starting my own online clothing store. At the end, I'm just not a fashion retailer. I'm a marketer. So for now, I'll just continue being a patron to other people's online stores.