DTox
Click on Vendor, hit sell
Right click on Mana Crystals, hit all
Keep clicking all
It will incrementally increase the number of mana crystals being sold past what you have available in your inventory
This also makes the "Price" column go into the negative
Also if you keep clicking All, eventually the Mana crystals will go negative in the vendor box along with the gold
When you click sell, you will get 0 gold from the vendor