Wow, it’s awesome that this design still has legs a year later!
As far as the spirit of the license, which is what really matters, if you’re making these to sell just put a “based on a design by Fiona Hopkins — http://bit.ly/MtgGears — CC BY-SA 4.0” on any label or something where you would also have your own name as credit. (That URL just links back to this thread.)
You don’t have to put attribution on the objects themselves.
If you do make any modifications to the mechanism (folks here have done great things improving the axle) and distribute those beyond personal use, you’ll need to make the SVGs available and have them under the same CC BY-SA 4.0 license (that’s the “Share Alike”) bit.
You don’t need to share SVGs of custom art for the gears or the back, though. Just the mechanism itself.