Is the questioner required to do anything for telling the seller that the device is "broken," even though she did not know the real reason at the time, and is this considered concealing the truth?

You did not have the right to take another device from the seller. Your obligation is to return the other device or pay its price. If the seller cannot be reached, you must repent and give charity equal to the value of the device on his behalf, while guaranteeing it to him should he appear.