On the side, I create t-shirt and vest designs then sell them online and offline. They're usually designs based on London and urban life, but there are geeky aspects too!
At first, I tried selling t-shirts at the occasional market stall. Now, remaining designs can only be purchased online.
To save on the printing process, I now put new t-shirt designs up at third-party online marketplaces (RedBubble, Society6, Threadless).
But DIY was more fun and it's fee free (so cheaper mark-up on price)! :) To see, check out my shop.