I use redbubble and Teespring both. Stickers are pretty popular thing for people to get from my redbubble, while Teespring is good for shirts. Use Premium SHirts from teespring, not basic. They are better quality and the print lasts longer. I tested it, and my premium shirt is almost 3 years old and still has the image on it. Regular shirt's art wore off crazy fast.

https://www.redbubble.com
https://teespring.com/

Upsides
Redbubble: Offers a large variety of content you can place your stuff on like stickers, mugs, puzzles, blankets even, and sketchbooks. The stickers are good (tested). redbubble is a well known place for people to browse.
Teespring: Largest Profit margin off all the sites I have used. I've gotten Tshirts from their the Premium shirts last a long time and come in a lot of colors. I've also gotten the masks from here, and find them comfortable to wear. It's actually one of my most comfortable masks I own. OH AND you can do Custom Discount codes whenever you want.
Both: You can shelve old designs.

Downsides
Redbubble: You get barely any profit from your content. Even Tshirts. Discount codes are entirely dependant on the site handing them out. you can't made discount codes yourself.
Teespring: Quality checking the products your selling before launching is a good idea. A regular Tshirt is a Bad Product to sell.
Both: No Fanart. Copyright. Just a heads up.

ya can always do so :) i use redbubble to sell stickers etc. for stuff like perlers or anything else i use etsy :). can always look around to see what is best for you to sell on :)