I think there is nothing wrong with the English term “sphere of influence” in itself. But, the problem is that it usually means “勢力範囲、影響範囲、影響圏、縄張り, etc.” in Japanese, where a nuance of “商勢圏” is missed.
As described in the original Japanese text, it says “商勢圏”. In that sense, “area (or sphere) of commercial activity (or commercial activity area [or sphere], or as the case may be, ‘sales territory’ or ‘commercial sphere’)” is more suitable expression in that it refers to a trading area in which the company is capable of having a competitive edge over their competitors.