in cases of theft it usually depends on what the designs are for or quality of the art. many cases i heard about were fanart of some popular series or just anime/cartoon aesthetic stuff that could easily be reproduced and sold. i mean it's a concern regardless but so long as you get your original work and name out first it's harder for folks to steal or repicate (and you can always file a DMCA takedown)
I haven't got a store on redbubble or any other print on demand site but I do have a ko-fi shop
ironically i'd had past concerns about theft when i'd spotted another online vendor under the name vibrantfox elsewhere on the web but as it turns out there was no theft just had the same name. i think i lucked out though since the domain is for sale (vibrantfox,com) but the buying price is steep so i think i'm more likely to go for a .art if ever i decide that i want a unique site
