Redbubble is Print-on-demand (POD) platform.
Most popular platforms are : Merch by Amazon, Red Bubble and Etsy.
I tested Redbubble with few designs. Quality of your designs, keywords and descriptions could help but I suppose that is far from enough.
There are lots of strategies to get better position in search results. Let's say quality of design is not one of those. Social media, advertising or quantity over quality are (I suppose) strategies to get ranked.
First you need good position on platform search results and then customer have to decide that your work is good choice.
Ex:
I sell my joke work over real work. 15 designs bring me 1-0 daily visitors and 1 sell in few months.