Is their another profession where a seller or buyer and their professional representative have directly opposite interests?
Plenty. Every profession has potential conflicts of interest based upon the economic incentives.
In civil litigation, an attorney on contingency often has a greater desire to settle a case prior to trial in order to get a guaranteed payout in settlement (and recoup their costs to date) than does the client who might want to go for maximum theoretical value at trial.
An accountant paid a flat fee to prepare taxes is incentivized do only the bare minimum to get the assignment done, and of course to deal in volume rather than quality overall. Same with doctors in managed care. Same with lawyers who (on the insurance defense side, more and more nowadays) get fixed fees.
The commission at least encourages the realtor to maximize value. I don't mind that so much as I do the dual representation that realtors get into, at least out here in CA. Then you
do have a conflict of interest at work.