As a t-shirt and graphic designer who creates images and graphics to go on all types of products I am always trying to get my design up on as many websites as possible to make more and more money all the time. Most designers that are serious about graphic design know that time is very valuable and when you can save yourself hours of work it is only logical that you would want to do so as long as it does not compromises your work ethic. This is why sites like Creative Market are so great. As designers we can find fonts and other design elements that you can use in your designs instead of creating everything from scratch all the time. When you need a graphic of a bird, or a dog you can go to a site like creative market and look for graphics that other designers have already created that you can purchase and incorporate into your design thus, saving you time.
Well as I was searching for graphic elements one day I noticed that there were complete t-shirt designs uploaded by a select few designers on the site that do t-shirt design exclusively. That gave me an idea to do the same as a way of getting more money out of my designs. Not only am I uploading my designs to various websites to sell on different products, I can now also sell the design itself to others who are doing the same as I am. I know the number one thing you think about when you hear this is that a million other people as well as the designer such as myself will be uploading that image to all the same sites you would be uploading to but that is the trade off you take. I have several designs that I sell on creative market that I have never used on any print on demand sites myself but it isn't always the case. I think the thing you should look at is that, yes you are not the only one that has that design but that does not mean you can't sell it also. It is really all about how you use the design. For instance you can change the color or even modify the design completely so that it is unique only to you. Also if you don't even bother changing anything about it it always comes down to who can use keywords the best and do better job of marketing. If your skills are better than most when it comes to keywords and marketing it doesn't matter if a million other people have that same design. You will be the one that buyers find and purchase from because of your skill.
Nonetheless, if you are a design and you want to branch out and make more money from your designs I suggest you tap into sites like creative market to make more money off of the designs you already have. The thing I like about creative market is that instead of getting paid you can use the money you have earned to purchase other design assets that you need to create more designs. I buy a lot of graphics, fonts, brushes, actions, textures, and other things from creative market to make my designs with. In fact, I sold some of my designs and turned around and bought someone Else's designs with the money I made. It makes for a great way to basically get free designs depending on how you look at it.
No matter if you are a t-shirt designer or simply a designer in general. I suggest you look into selling some of the assets you create on Creative Market to add another stream of income to your finances.