If you make a low enough offer that they can't accept, they will tell you and they have told me in the past that its because its below the sellers reserve. Whois may also give you a clue as to who owns the domain.

They have fairly decent margins for discounts - I used to get marketing emails offering 20%+ off prices in the past.

Just look at the whois. If the admin email is brokerage@buydomains.com, they own it.