Ok, so first off, Ali has a bad reputation. There are lots of bad reviews everywhere, people say things are too small, don't fit, never arrived etc.
First off: Check the sellers rating and how long they've been on Ali. If they have 3 or more Diamonds and have been on there for over a year, and if their items actually have "x amount sold", then usually, it's absolutely safe.
Secondly: Read the descriptions. Most sellers feature measurements which I have always found to be very true to size.
Thirdly: Read the text! They all get the items from factories so it takes 8 days or more for them to ship your item. If you want free shipping, it will take a month to receive your item. This is just how it is, plus it's FREE and the item you are buying is cheaper than elsewhere.
And lastly: TRUST YOUR INSTINCT. Yes, an ihpone 6 for 40 usd will most likely be a scam. But if you want to buy clothes, shoes, accessories and bedding, this website is top class.
I have always received my items, the customer service was reliable and great and I can recommend it.