Electronics – either buy on Taobao or find out on Taobao what the price should be first, then go to a local seller. During a recent computer crisis, I also discovered through Taobao that the upper floors of places like @world and digital plaza are full of wholesalers who sell the big ticket products for a few hundred less than the booths downstairs.
Perishable Food, like dairy or meat – use Taobao to find best prices, sort by local seller, and make a specific appointment for delivery so your package is not sitting with the security guards for a day. Also avoid buying in really hot weather.
Clothing – Some sellers are gold; you can tell them your height/weight or other measurements and they will be able to say if something will fit you or not. You can also ask the seller to take extra photos of the exact item you are considering and send them to you via chat – I thus avoided getting some very fake Birkenstocks last summer.
There is also a lot of junk on Taobao, so it pays to only consider the 7-day unconditional money back guarantee items and then pay the .5-1 kuai extra for return shipping insurance. Really cheap stuff, underwear, and most second hand items will not offer the guarantee, so you know, caveat emptor.