The Trade-O-Mat is a block added by Industrial Craft 2 used to make safe trades when connected to a chest (preferably a personal safe).
Once a Trade-O-Mat has been placed, the first person to right click on it becomes the owner, meaning that only them and server ops will be able to remove it or change what it accepts for payment and the product it sells.
Other players right clicking on the Trade-O-Mat will see a GUI with two slots. The top slot (with an up arrow) is for "payment", and the bottom slot (with a down arrow) is where the purchased item(s) will appear. The Trade-O-Mat GUI for owners.The owner will see 4 slots, two top and two bottom. The top and bottom on the left are where the owner specifies what will be dispensed, and what the price is. The top and bottom slot on the right are where the Trade-O-Mat can be used normally, even as the owner. The number of trades that have been made at that Trade-O-Mat will be shown numerically, as well.
To set it up, place whatever you would to accept as payment in the left top slot, and whatever the Trade-O-Mat will give out in the bottom left slot. The numbers in the stack matter. For example, putting 2 Gold Ingots in the top left slot and 20 Redstone Dust in the bottom left slot will set the Trade-O-Mat to accept 2 Gold Ingots as payment, and in return to give out 20 Redstone Dust. The items you put in the left slots to set the item being sold and the price will stay there until changed. Multiple sets can be purchased at once, so someone who uses that Trade-O-Mat and puts in 6 Gold Ingots in the top right slot will get 120 Redstone Dust in the bottom slot.
A smart player would realise the profits in this machine. If you sell batteries for a price of 5 Diamonds (uncharged) and charged batteries for 7 diamonds, when they went to sell their used batteries for 5 diamonds, basically they are paying 2 diamonds for energy. Be creative, the possibilities are endless!