Is the frame made from heavy duty galvanised steel? That makes it suitable for all year round use. Ask the weight of the trampoline in its box. its a good guide to the strength of the frame. Generally the heavier, the better
The better the spring, the better the bounce. Simply ask how many, how long and how strong? The low cost springs used in cheaper trampolines lose their memory especially if used by adults. A trampoline spring that will not spring back to its original shape is soon a disappointment and of little value.
The mat is should be soft and smooth to touch to ensure it doesn’t hurt your child’s skin. Ask how the springs attach to the mat. A D ring system of attachment, rather than hooking the spring directly into the material, will greatly extend the life of the mat.