The steel
Warning: 'Stainless', 'surgical stainless', or 440 on anything longer than a dagger.
Sound: A named carbon steel — 1055, 1065, 1075, 5160 — stated plainly in the listing.
Stainless alloys trade toughness for corrosion resistance. At sword lengths they are brittle, and a brittle blade under load is shrapnel.
The tang
Warning: No tang stated at all, or a handle that twists when you grip the blade and turn the pommel.
Sound: 'Full tang' or 'through-tang, peened' — stated, and visible in the construction.
The rat-tail tang — a thin rod welded to the blade and hidden in the grip — is the most common failure in cheap swords. It breaks at the weld, and the blade leaves the handle.
The heat treat
Warning: No hardness stated, no maker named, no warranty in use.
Sound: A stated hardness (roughly 50–58 HRC for most swords) or a house that answers for its work in writing.
You cannot see a heat treat, so the dishonest skip it. A soft blade bends and stays bent; an over-hard blade snaps. The treat is where cheap swords are truly cheap.
The price
Warning: 'Battle ready' at $39.99 with free shipping.
Sound: Functional carbon-steel swords from honest makers generally start around $150.
Steel, heat, and labor have floors. A price below the floor means a corner you cannot see was cut.
The words
Warning: 'Battle ready', 'ninja', 'samurai sword set of 3', glowing reviews with no specifics.
Sound: Plain specifications: steel, hardness, tang, weight, balance point, maker.
'Battle ready' has no enforced standard. Specifications can be checked; adjectives cannot.