Short answer: anything where used condition can't be safely verified — car seats, helmets, cots, mattresses — plus anything missing required safety marking, like unmarked toys or unlabelled upholstered furniture.
Most of these have a proper home somewhere — council reuse schemes, Freecycle, specialist recyclers, or manufacturer trade-in schemes. Search any item name in our directory search bar and we'll flag it with the right next step automatically. Working through a whole clear-out with a mix of all of these at once? Hands-on sorting help can save you the guesswork.