Mapnik does not show "shop:electronics"

Additionnaly to other answer about OSM's mapnik policy, you can use other tools for viewing the wanted data. For example, you can use this xapiviewer to see such shops in Berlin.

As Pieren said, "why is XYZ not rendered on the main map" is a question that is asked quite frequently. This is a question from someone from Sweden about "shop=alcohol". Frederik's answer explains where the code is and what the mechanism currently is for requesting changes to it.

However, it's worth expanding a little on that. This mailing list thread discusses the current problems with main map style and how easy it is (or not) to contribute to it. In some cases the problem is that the data currently isn't in the rendering database so adding it requires more than just a style file change (I don't think that that's the case with "shop", though)

With regard to the second part of your question "in Potlatch you cannot select this shop type", have a look at this previous question. The procedure's similar to the main map style - log a bug in trac using your normal OSM login, assign it to the correct area (in this case "potlatch2"), and offer whatever help (icons etc.) that you can. In the case of Potlatch new trac entries are forwarded to a mailing list where they can be discussed further.

In Potlatch you can, of course, always select "advanced" mode and add whatever tags you like.

Please use the 'search' function to find previous questions about the same topic. E.g. https://help.openstreetmap.org/questions/1107/can-i-influence-what-gets-rendered-in-the-main-mapnik-layer