As someone who buys domains I can tell you how the big brokers operate.
BuyDomains: You fill out form online, they email you price or call you. They will reach out to you by phone here and there, never aggressive. In most cases you will get a few emails (automated) about the name you asked about. HEre and there they might let you know about a big discount for buying the name.
DNS: You fill out form, they might call you or not but they put you on the email list for that name. The emails you get will be anything from asking a time to call you to letting you know another person is interested in the name.
AquireThisDomain: they will email you a few times asking to counter offer, never had calls.
Marchex: They will email you back once with a price but you wont hear from them again
MostWantedDomains: they have (at least in my experience) pre set limits as to what they want for the name. Those are min. price and not necessarily the price, they are not aggressive.
Every domain is unique, only you know how many offers you ever got on that name and only you know realistically what the chances are anyone would offer more than the current offer.
Many emails you get about domains are people who are excited now about it, it could very well be he wont buy the domain name now for even 50 bucks.