Tiered taxation system? Maybe tax the purchaser too?
I'm not sure if any of this solves the problem if and when bots multiply. Unless they are kept to a minimum (bots in small numbers have very little impact on a millions-of-people economy), you'll get massive wealth inequality, and the only viable solution for the have-nots will be to bot as well.
But yes, if bots are kept under control, there are lots of viable solutions. Doubling repair costs would be a good start, for example. Maybe making items bind on equip or to account. Make a new crafter like the BS/Jewelcrafter to sink more gold in to. Any or all of these would help, and aren't especially complex to implement.