When you talk about Mrs Weston slipping from one form of a commodity to another, what did you mean? The definition of commodity I am used to is a good that is interchangeable with other commodities of the same type. For example, a bag of standard flour is fairly interchangeable with another bag of standard flour. A pair of shoes made to your specifications is not a commodity. And people are not commodities, we all have our own personalities and quirks, even more than a pair of shoes.

Neither Emma nor Mr Weston appear to have regarded Miss Taylor as a commodity. In chapter 1 JA says:
Sixteen years had Miss Taylor been in Mr. Woodhouse's family, less as a governess than a friend, very fond of both daughters, but particularly of Emma. Between them it was more the intimacy of sisters. Even before Miss Taylor had ceased to hold the nominal office of governess, the mildness of her temper had hardly allowed her to impose any restraint; and the shadow of authority being now long passed away, they had been living together as friend and friend very mutually attached...
I don't know about you, but "intimacy of sisters" and "friend and friend" sounds like their personalities worked well together.

And as for Mr Weston, why would he be thinking of Miss Taylor as something like a commodity? If he was merely pursuing a rich wife then I could understand this view, but JA tells us in chapter 2 that Miss Taylor was basically portionless. So we can therefore conclude that Mr Weston was not marrying for money. And JA says that It was now some time since Miss Taylor had begun to influence his [Mr Weston's] schemes. (chpt 2). This sounds to me like Mr Weston was not merely looking for any wife, but was attracted by Miss Taylor over a long period of time. In that case, what could Mr Weston be interested in if not that he saw something special about Miss Taylor?