I bought some pants (AmE). OK [This is said]
I bought some trousers (BrE) OK [This is said]
I bought a pair of pants. [note the a unless you are responding to someone, who says something like: How many was that? Then, we say: I bought one pair of pants or trousers. The same with all things like this: I saw a car I liked. Not: I saw one car I liked, unless you are clarifying] OK
I bought a pair of trousers (Watch that a :). OK
I bought two (or three or four) pairs of trousers or pants.
That's the skinny on this. I bought two trousers is "unacceptable".
Scissors and goggles also follow this exact pattern.