Wands of Equal Trade - great for builders
If EE3 is not installed, you can make clay from redrock or river bees, or a factorization processing line. It's useful in GT since you can make it into energy and for making bricks, but difficult to get in large quantities. Try selling both clay dust and clay balls. Depending on people's needs, one might sell better than the other. If you have EE3, don't bother.
Enchanted books - It's hard to automate in mindcrack, although I think you can use turtles to collect XP and enchant books. (DW20 did this in one of his let's plays) Most of them would be fodder(could eventually be used in a GT magic energy absorber or such), but the right ones would be worth a lot.
How about a potion shop? Potions of fire resistance, Regen, etc. Those would be very beneficial to a lot of people if they could be purchased easily.