Maybe this is too specific but I would say it's the same as a pair of trousers/trousers. If I'm speaking generally, I'd use pair. I'm going to buy a pair of trousers this afternoon/Does anyone have a pair of scissors I could borrow? //// My trousers are soaking wet from the rain/I left the scissors in the kitchen.

"A pair of scissors" is the general term. You would probably say "the scissors" if they are talking about specific scissors but you might also say your, his or my scissors (this list is not exhaustive). For example: where are the scissors you were using? Where are your scissors? Have you seen my scissors? Do you have a pair of scissors? It's always "two pairs of scissors" and never "two scissors".

This one is tough. There are certain times when one construction sounds better than the other. For example: - you would say "Where are the scissors?" but not "Where are the pair of scissors?"; - you would say "Do you have any scissors?" or "do you have a pair of scissors?" but "do you have scissors?" doesn't sound right; - you would say "Did you find the scissors?" but probably not "Did you find the pair of scissors?"; - you would say "Can I buy a pair of scissors?" and "Do you sell scissors?" but not "Can I buy scissors?".

A pair of scissors