It's too bad the previous owner di not answer your questions about the status of the grill. I hope you got it at an excellent price in case you have to spend money on replacement parts to get it operational.
Since you have no idea how the grill was treated in its previous life, I suggest you purchase some fresh pellets and put it through a "seasoning" session. First remove all the grates and heat deflectors and make sure any residual grease and grime is removed from the inside of the grill and the removed parts.. The grill does not need to be showroom clean, but you want to remove any excess that might start a grease fire. Put the grill back together, fill the pellet hopper at least 1/2 way, prime the auger, and ignite the fire. Set a temperature around 225 F as that is a common smoking temp. Once the grill has come up to temp and stabilized, let it run for 15-20 minutes. Then check the temperature inside the grill with an oven thermometer you know to be accurate. If the temperature is within 25 degrees, you can live with that by adjusting the thermostat higher or lower as needed to achieve the desired grill temp. If it is off more than that, you might need either a new temperature probe or a new controller.
After running the grill a while at the lower temp, start ramping it up to 350 and finally all the way to maximum. Check the temperature using your oven thermometer so you know whether the grill probe is working. Let the grill run a maximum for about 1/2 hour to burn off any residual grease. Add pellets if the hopper starts to run low. After the test run, follow the shutdown sequence for the grill and allow it to cool completely.
If you are able to complete the seasoning run successfully, you will be ready for your first cook. If you encounter problems along the way, either come back here or call Traeger customer service for assistance. You should be able to find any repair parts you might need, but hopefully that won't be needed since it sounds like the grill did not get a lot of use.