Does that also mean that we need two separate gismu for "to buy" and "to sell"?
This is a good question. I considered the possibility of wanting to have this since "sell" is somewhat of an agentive verb in English (sure, a grocery store selling something isn't particularly agentive, but a car salesman selling something is), which if {te vecnu} is really "buy" rather than "is sold" isn't reflected in the semantics of {vecnu}.
At any rate, this is a marked inconsistency either way; if {selsumji} is adequate for subtraction then {selpilji} is adequate for division.