As DarkBanner said, TE as well as mid level to max level units are all considered currency depending on what others are looking for.
At level 40 to max level 4*s they are usually considered 2 TE (level 40) to 5 TE (max) and eaten as feeders into higher starred cards. Selling them fresh will only get you either 1TE or 2 for 1TE these days.
The cheapest 5* is 8 to 10 TE.
Just to give you a bit more perspective on what you are aiming for. I hope it was helpful and good luck!