of course, as a consumer i would be more than happy to sell my lawnmower, fish or whatever. but as a retailer, i would be annoyed. if this were the case it would simply be deemed bad business ethics. if i were the retailer, and someone walked into MY shop (which I pay rent, power, rates etc for) and someone else was selling THEIR goods to one of MY potential customers, i would not only find that unethical but outright rude.
of course this depends on the circumstances, if the 3 of them were in conversation and the sale meant it worked out nicely for all parties involved, this would be a different story. again, depends entirely on the circumstances.