Good example are industrial switches which locally are 20x the price I can buy the exact same product directly from China including postage.
There's a four-floor building full of electrical / electronic stuff right in that same area. You may think you died and went to heaven
DMF Tom's suggestion to bring prints of photos is a good one. Just point to the picture ..
I wouldn't put too much hope in finding certain things tho, it's more like a huge flea market. You'll probaly find a lot of stuff you weren't looking for, and not find things you want to find. Imagine a DVD store where they just pile everything into boxes, helter-skelter ....
I am stopping in Shanghai ...
The one thing to be careful of in that Nanjing Lu pedestrian street area is, it's where the predators go to hunt foreigners. It's not unsafe, nowhere in China is, but
1) Pickpockets. Lots of them. And they are good. Money and telephone in front pants pocket, please.
2) Whores. Worst place in Shanghai to get a whore. Suggest you shop elsewhere
3) If you are approached by a couple of cute college girls who invite you to practice English, see an art show, partake of a traditional tea (and cookies ?) ceremony, unh-unh. No way. When the bill for the tea comes it's like 10,000 rmb and you'll find the doors locked and the cute college girls disappeared.
If it's too good to be true ? Don't do it. That area is full of scammers.
And taxis are easy but a lot of them are dishonest, so learning to use the subway is good.