The most important specification is the format. A keyed cylinder, combination padlock, digital keypad, hasp-compatible padlock, lockbox or safe all solve different problems. A well-priced product is poor value if it does not fit the item you want to secure.
For door security, check the lock type, cylinder profile, handing if relevant, fixing method, included keys and whether the product is intended for internal or external use. For padlocks, compare shackle material, shackle clearance, body width, key or combination operation and whether the listing describes outdoor suitability.
For safes and lockboxes, compare external and internal dimensions, locking method, mounting options if listed, weight, capacity and whether the product is intended for keys, documents, small valuables or general storage. Do not rely on product photos alone, as a compact safe can look larger online than it is in real use.