You can always try Amazon for used garden tillers for sale.
There are more,and more private parties selling their stuff there,and a person looking around can save a few bucks buying a used one.
However,odds are you'll be right back here looking for used parts,if it's a specific model,or you'll have to do your home work locating spare parts online for one.That's if you're repairing one yourself.
When it comes to used garden tillers.I just worry,because a person really don't know what the previous owner put it through.If a person can get a used one of a friend,or neighbor,that's another story.
But buying a used garden tiller online is a bit to much for me.The price you pay,it's better off just getting a new one.
You have a warranty,a manuel,and information comes with it on repair,and ordering parts.
So check Amazon.They are known to have used tillers for the garden on sale.Free shipping is usually included.