"Greengrocers" is a group of greengrocers; "greengrocer's" belongs to the greengrocer; "greengrocers'" belongs to a group of greengrocers; a greengrocer is the person who sells your groceries. Presuming your greengrocer's establishment is independent and not franchised by a group of greengrocers who call their shops "Greengrocers'", I'd be inclined to refer to my greengrocer's premises as a greengrocer's. Cue the grammar Nazis.
I'd agree. You're going to a shop belonging to a greengrocer, therefore, it's the greengrocer's. It could be the same premise as, instead of saying "come to my house", you'd say "come to mine", and when speaking about third parties, "Let's go to Rickie's." "I left it at Kelvin's." rather than "Let's go to Rickies" or "I left it at Kelvins" - the house belongs to them, so is Rickie's/Kelvin's, similar to the establishment of the greengrocer. Unless the shop is owned and run by more than one greengrocer, when it may be greengrocers'...

