I'm a brand new player (today is day 4 or 5; I forget which), but I'm trying to help out other players, as I've gotten lots of help so far in my small, brand new companies, so I'm trying to return the favor.
The following link is from a site you can join for free -- no strings attached as far as I know. It will let you look at an electronic guideobook a game player of Virtonomics made, that covers your question in depth.
To summarize, go to the top screen bar, click on the very top bar labeled "Game World," go down one line to "reports," and click on that.
This will bring up the City Retail Report for every city in your game (you have to do it separately if your in either Mary Realm or Lein Realm.)
So you choose a country, then choose a region, then choose a city, and finally, choose a class of products, such as foodstuffs or groceries or perhaps clothing or electronics if you have lots of money to pay for purchases.
Look for an item with an "E" near the top,next to the Market Development Index. This means that item isn't that saturated with local store sellers or player store sellers, in that city.
Then select one of each of the icons for items in that class.
This will show the total number of items sold in that city for that item, and in the far right, the average price sold by local (virtual) game sellers, as well as real world players.
It will also show you the average quality, and brand.
Then you go to your retail store, select "add supplier" and filter out only suppliers that have at least "1" of that item, and that have at least "1" available.
As I said, the guide book does a really good job of explainging all this. but I hope my summary of it helps you out, because you want to sell items at as high a grosss margin as you can in a city that isn't overly saturated.
