I hadn't heard of this place before, but one of the posters on MSE mentioned them the other day.
If you're familiar with websites such as Folksy and Etsy, then MISI is much the same. You pay 20p to list an item you've created and it stays on the website for 12 months, unless it sells. If it does sell, you pay 3% commission, which seems pretty reasonable to me.
Needless to say, I've signed up and I've just added my first 5 items - a selection of handmade bags. I have lots more to follow, so please do pop over and have a look at my shop: