1) Domain appraisal remains a difficult to standardize process. Most sales are not reported. The reported sales may or may not have had the benefit of outside advice about value, i.e., they weren't a true arm's length transaction (where each party has access to the same info and has the same bargaining power).
There was a time when you could post domains at various forums and get a good deal of input from experienced domainers. I no longer see that happening, likely in part because some much junk was being posted for appraisal and party because domainers have better things to do with their time. Still, you could try . . @ DomainState, DNForum, NamePros, etc. . . . just not here.
2) Brokers? Meh. Only the very best domains and often to other domain speculators and not to endusers. 1/10th or far less than enduser value. Sites? All the usual: Afternic, Sedo, etc. Auctions? Ha ha ha.
The "acid test" (reality test) for domains that are assumed to be great domains is "How many unsolicited offers have you received in the past years?" and, if you have "What is the best offer you received?".
Everthing else about "great domains" is just dreaming.