There is no reason why a skateboard or a yo-yo cannot be classified as Transformers toys. Semantically speaking if you can play with it then it's a toy. The word "toy" is of Germanic origin and just means "plaything." Heck, Toy Story Four seems to be exploring what one considers a 'toy' with the character of Forky. According to what's been shown in the trailer, Forky is literally a plastic spork that the kid has made into a toy, and thus it has come to life through the power of the child's imagination and love for the toy that she's made. But Forky rebels as he clearly does not view himself as a toy but rather as a disposable utensil.

But I digress... In answering the question posed by the thread topic: "What is your definition of a toy and of merchandise?", I'd say that the general consensus is that action figures count as Transformers toys while other stuff counts as merchandise. As with all systems of classification this was always going to be highly subjective. Even scientific classification systems can be subjective... like remember when we were kids and Pluto was a planet?