Part of it is just getting a feel for Chinese goods and using that to determine what is a good deal or what is a scam. I’ve worked with a lot of Chinese suppliers for a long time so I have a sixth sense for their bullshit.
Generally I avoid anything that could be a safety risk (e.g. Mains power) , and keep an eye on the photos to see if they look too good to be true. Official stores are good, as are items with many purchases and reviews that include pictures.
Paul’s online math notes are by far the best resources out there for the first few university level math courses.
https://tutorial.math.lamar.edu
It is a great shame that his linear algebra notes were removed years ago.