We never know but to become wandering merchants you would need some life skill on seeing great products, sejun probably teach theo after he got scamed by scaram
from what i remember, theo has always had that talent of finding rare things plus his skills of asking for discount lol and sejun noticed (theo himself didnt even know) that’s why sejun told him to bring anything that catches his eye
what up with Theo. every time he goes out he brings back things that are rare or lost treasures. Is he some kind of item drawer or something??