If I recall rightly from Lynne Truss's Eats, Shoots and Leaves: for "of plural grocers", it's definitely " grocers' " -- plural-with-s followed by apostrophe-s, doesn't happen. (She mentions also in her chapter on apostrophes: that some well-established institutions express their names in a way which, apostrophe-wise, is technically wrong; but, with the force of long tradition behind them, "their way" is accepted: example, a certain big London hospital -- its name, concerns only one St. Thomas; but damn that noise -- it calls itself St. Thomas' .)
Re tradesmen tending to misuse apostrophes: Truss ascribes this to greengrocers (potato's; banana's; etc.) rather than plain grocers. Although she's normally pretty merciless about any punctuation infraction; she writes quite leniently re greengrocers in this respect -- in part because she sees them as "horny-thumbed people who do not live by words" (condescending, or what?).