Most problems are visible before you pay. Look for these.
Specs that do not fit the price
A very low price per Wh, per GB or per watt compared with similar products is a warning, not a bargain. Work out the unit price and compare.
Fake-capacity storage
Micro SD cards and flash drives are sometimes sold with a printed capacity that is not real. When one arrives, test it with a free capacity-test tool before you rely on it.
Numbers that contradict
A title that says 65W while the spec table says 20W, or photos that show a different model, are signs the listing is not carefully made.
No traceable brand
For batteries, chargers and other high-risk products, an unbranded listing with no store or distributor to trace is the main reason we exclude a product.
Unrated claims
“Waterproof”, “fast charging” and “military grade” with no rating or standard named tell you nothing you can check.
If you already bought something that looks wrong, stop using it if it gets hot, swells or smells, and ask for a return through the marketplace. This is general advice, not a diagnosis of any product.