Player trade terminals can be placed on any ship/station, and do not require unobtanium. When adding them to your design, it should be noted players have to board your ship in order to buy or sell things from the terminal, so it's best to put them in their own room to keep players from walking all over your ship.

Trade terminals can buy and sell every resource and ever tool/item in the game. When you select a resource or a tool for the teminal to trade, it will automatically fill in a suggested price, but you don't have to stick with these prices. The terminals can both buy and sell resources, but you can set them to only sell or only buy. If you want the terminal to be able to purchase resources from the start, you'll need to supply it with Units. Every terminal has it's own Unit balance, which all transactions go to. This also means that you have to manually collect the unitss from the terminal every so often, and if you modify or salvage the ship with terminals, you'll lose all Units stored on those terminals.

Trade terminals have some additional not as obvious use cases beyond trading with other players. The first key thing to know is that NPC trade drones will interact and trade with the terminals. They will only trade with the terminals if they can make a profit somewhere in the same star system (and they can't trade tools), but if you're willing to spend the Units, you can use this to have the drones continually supply you with fuel, ammunition, oxygen, and other resources. Note that if the resource you're trying to buy isn't available from any NPC stations in your current system, the drones won't be able to sell it to you.

The trade terminals have another far more interesting use, but because you can't stop the drones from interacting with your terminals, it does carry with it a risk of the drones stealing all your resources if you're not careful.
When you trade with a terminal, you can select a ship you own nearby for the terminal to buy resources from or sell resources towards. The terminal does not care how many resources you move, and you can set the buy/sell prices of terminals to 0 without issue. The upside of this is that you can instantly teleport massive quantities of resources to nearby ships that you own. However, because the drones also don't care if the price is set to 0, they will use terminals set to 0 to steal as many of the resource as they can carry.* So if you do make use of the trade terminal in this way, you have to make sure there are no trade terminals nearby; and after teleporting your resources, set the sell price back to a more normal value before the drones warp in.

*This may have been fixed, I'm not certain. Set it to 0 at your own risk. That said, there are other ways trade terminals could be useful if you could toggle whether or not NPC drones can interact with them. For example, having a separate room that only members of your fleet can access, where they can buy resources at a discounted price. Please petition the devs about this on occassion, as history has shown SP will most readily fix/add features if you pester them for long enough. :P