So Jormagund is a level broken 4* Caster Poison, while Tugba is a 5*. If you end up leveling breaking Tubga, she would be max level at 120. In a way unit wise with stats, the 5* is always better, but Jormagund has good art.
I think some people are trying to sell the 4* for higher prices due to art, or it being level broken, etc.
In the end the best determination is what appeals to him, etc, but it is a legit offer. I hope that helps.