AliExpress

I used them because they were the only possible suppliers of a particular plug-in transformer/jack plug combination that I needed. They were so cheap, postage included, that I was quite happy to write off the small sum involved if they proved to be a failure. In fact the item was delivered by Bpost within a few days and has performed satisfactorily ever since.

The main disadvantage compared with Amazon is that you aren't buying from Aliexpress, you're buying from a small company or individual in China, so if there are problems, it won'T be as easy as Amazon, where returns are extremely easy.

They do have a complaint resolution service, but I have heard (no personal experience) that it can be very difficult to get money back.

As with Becasse, i've bought from the platform with no problems, and the prices are dead cheap. However, be aware, you can get hit for import duties and VAT.

I order things regularly at Aliexpress. I have ordered mostly small things like plugs/Adaptors/Decorations/GSM accessories/Bathroom accessories etc. My feedback:
1) Takes ages for the product to be delivered (have waited for 45-60 days) --> but then you don't pay any delivery charges.
2) Also because they offer free delivery, the packaging sometimes can be very bad. Sometimes when I received the product, it was either crushed or broken
3) Quality is not always good but again the prices are so low, you would still save money if you buy it again (if first one breaks down) then spending almost 4-5 times the price you would pay in Belgium
4) Biggest advantage (contrary to what CM wrote) is that they have excellent complaint resolution process. If the products are defective or damaged during transit or even delivered after what they call a purchase protection period or not delivered, all you need to do is to initiate a dispute on the site. These disputes are handled very promptly. For every dispute I have created, I have got my money back in 2-3 days. In fact the individual vendors are so scared of these open disputes that they reply very fast requesting to close the dispute or deliver the money. I never had to send back any of the products. I don't know how the experience would be if there was an expensive electronic item.

Conclusion, if you are willing to wait and want to buy "not so expensive" things, do go for it. Don't worry about the defective products, the customer service is really good.
If you want an expensive item or need the delivery earlier, Amazon is a better choice.

PS: Because it is so cheap, I happen to order things which I don't even need :)