I'll leave a note on the seller's boat, he can contact you here and you guys can hook up via email if you like.
The 57,000 asking price is normal for the age of the boat with trailer; it depends on the condition of both. The 300 HP is a big plus, probably not the original motor, hopefully low hours. Outdrive or shaft? If outdrive, find out when it had it's last major overhaul. They (Volvo) tend to detonate between 1500-2000 hrs. Diesels typically last 5000 hrs, 2-3 times as much maintenence on the second half as opposed to the first 2500. Steve can help you with valuation, he sells alot of used Blackmans.