In my next trip I have a long layover at Singapore. I'd like to make the most of my time there visiting cheap electronic gadgets shops (mainly chinese products). I've read that there is a building called Sim Lim Square where you can find electronic items, but everything I've read about it's refered to international trademarks (Nikon, Lenovo, Apple...) and (worst) the prices are equal than in Europe. To clarify, I'm looking for items like the ones you can find in webs like AliExpress or BuyInCoins when you search in the "electronics" tab.
Not sure where you've been reading that, but I can assure you they're quite wrong, Sim Lim Square sells every make of cheap electronics, so Chinese no-name brands and outright ripoffs are legion. Back in the day when iPhones were the new hotness and weren't officially available in Singapore yet, I remember seeing a plethora of "jPhones", "iFones", "iPhons" etc, all thinly disguised Windows or Linux. Some dealers aren't above selling you the real thing and slipping a clone in the bag instead, although the Tourist Board is cracking down on this pretty hard.
The general rule of thumb for exploring SLS is that the higher up and deeper down you go, the more esoteric the fare on offer. To generalize, the ground floor is mercenary camera dealers and is best avoided, while the first floor is mostly mainstream audio gear, but the second floor up is all IT. The "inner ring" stores facing the atrium are fairly mainstream (laptops etc), but the windowless "outer ring" stores behind them sell components, used gear, etc. The official category list is a good place to start hunting, most shops have their own websites. Hardwarezone is also an excellent forum for more info.
The obvious choices to get your electronics are:
While they do sell the big brands as you noticed, you should be able to also find no-name products for quite cheap. For instance I've been able to find generic iMac chargers at Sim Lim for a good price.
Those malls are organised in a lot of small shops, so it is very easy to walk around and compare prices. However, note that it's also quite easy to get ripped off.
In my experience you can also get cheap electronics at other places. If you have enough time I'd definitely have a look at Mustafa Centre, a 24-hour mall which has almost everything.
Another option are small neighborhood shops, which are located at the heartland of the many HDB (public housing) estate. Those are however often off the beaten track and not so easy to find as a tourist. If your traveling from Changi Airport the city, you might for instance stop at Bedok MRT station, where one such heartland is located and try to find some electronics stores. (not the mall, but you have to cross the food-centre towards the stores that are located on the street)
However you'd have to be very lucky to find a store that is cheap and good. For that reason, I do think your chances are definitely the best at Sim Lim or Funan.
That being said, prices of branded electronics are not necessarily cheaper in Singapore compared to Europe as you noticed.