If I think of the way I do my shopping now, I basically only visit 3
or 4 different shops or supermarkets in my neighborhood: Once in a
week I do a big shopping, either in "Handelshof" or in "Rewe", or I go
to the "Famila Center" (all German supermarket chains). During the
week, I may pass by a "Penny" shop - useful for smaller and cheap
items.

Most of the time, I buy my milk, bread, jam, soap, tissues, and so on
in one of the shops I usually go to. Only in rare occasions I need
something special where I want to go to a more specialized shop, like
to buy a music instrument.

If I wanted to buy an Indian sitar, I would google for music shops in
my city nearby, or look for items on ebay and the like, but for most
of my items I don't need Google. I know where to buy a toothbrush or
fresh apples or a can of beans.

So I would like to see the option to enter my 3 or 4 favorite shops
into my android phone (the ones mentioned above), together with their
coordinates. (A clever shopping list may for example note that you
strike out 80% of all items within an hour, so it may ask "We are now
at ... and this seems to be a shop. Would you like to add this to your
list of favorite shops?"). Next time I am in the same place, the
program will notify my if there are items on my shopping list, so I
will not forget to look at my shopping list.

I guess this would be already a useful feature for me.

The features you suggested seem a little bit more difficult: One would
need a standard how shops can present information about items they
have and prices, and bargains. Is there any standard way how shops do
this now? If not, I think we should just go ahead and define an
openintents standard for shops :-)

Note that the list of favorite shops could be implemented as a subset
of favorite locations? This would be very close to central tagging (by
tagging shops as favorite), but in any case one would need a list of
shops.

So I think the best starting point would be to create a new SQL table
for shops (as part of shopping), in which the user can store
additional information like opening hours. Then Friday night at 19:55,
you could easily see whether it was "Handelshof" or "Famila Center"
that is open until 22:00 (instead of 20:00).

table "shops"
 - _id
 - name
 - geoURI
 - image
 - notes
 - openinghours

something like this. Could there be more useful fields?