As part of my expansion project, I have also added stitch markers for knitting to my Etsy site! I will hopefully add crochet markers, soon, too. I've been thinking about making stitch markers for a while, and my biggest problem was deciding on a design. I am currently using closed jump rings as the primary ring.
Closed jump rings are really hard to find! I found one size at my local craft store, two sizes at Fire Mountain Gems, and one size at an Etsy store online. The main problem was that I didn't want sterling silver. Anyway, I'm giving it a go, so we'll see how it goes!
So far, I've made my mom sets in 3 different sizes, and offer 2 sizes online, large (fitting up to a US10.5 needle) and small (fitting up to a US4 needle). I've had a little interest on Etsy and a moderate interest at the Saturday Market I go to.
Both pictures here are for the larger ring.