It definitely is harder to get sales, but still very possible. The key is really niche research and listening to the market. I do most of my research on etsy. I scour etsy for designs that have over 10 reviews and make my own design. I do some designs myself, but I also have a designer who I pay $3 per design.
The thing to realize is that you need a good portfolio of designs to make decent money. If you don't have 1000 solid designs then you probably won't make that much. One design unless you are really lucky, won't cut it. I would guess that only 30% of my shirts have ever sold.
