Correcting other people's mapping

If you find outright errors (like the closed post office) in other people's mapping, correct them without hesitation.

If the issue you have is something debatable, and/or if you find that the other person has only just added the object in question, it might be a good idea to talk to the other party to find out what they had in mind. An easy way to access the history of any object is to go to www.openstreetmap.org, zoom in, activate the data layer, then click on the object in question to retrieve details, and then click the "history" link. There is a (newly created) description of how to activate the Data Layer on the Wiki.

There's also a number of data checking tools that allow you to inspect data and find possible problems with it. Jonathan Bennett's OSM book has a chapter on data checking tools which you can read for free.

This answer contains material from comments by Andy Allan and Jonathan Bennett.

One easy way to find history is to select the POI in Potlatch, then press 'H' for history. The drop-down menu will show you everyone who's edited it. If you select an entry then click 'Mail', this will let you send a message to the user in question.

At the risk of stating the obvious, if the former post office is now just a shop, I'd change the tagging to say that it is a shop.

If it's now a private house, speaking for myself, I'd be tempted to leave the node there, remove the tag that says "amenity=post_office" and add a note that says that it used to be a post office but isn't any more (just in case someone tries to re-add it from some old notes that they might have). However, depending on the nature of the "former feature" it make make sense to delete it - if you live in the village and know the story you're probably the best to judge what's appropriate.