Is it permissible to sell a computer device for 2500 riyals, knowing that its purchase cost was 2000 riyals from another city 150 kilometers away?

If you are acting as an agent for a man in purchasing a device, then it is not permissible for you to take any additional amount. Otherwise, you would be consuming others' property illicitly, as Allah Almighty says: "And do not consume your properties among yourselves wrongfully." It is only permissible for you to take what it cost you to deliver the device to him.