I don't know what the Florida machine market is like, but location can play a huge part in pricing. Small knee mills don't seem to come up for sale that often and if priced right they move quickly. I have seen a few older import mills similar to that one which were asking $2000-2500 in well used "as is" condition. The fact you know the history of the machine should play a part, this isn't a random CL ad, how did this person care for their tools? If they took very good care of their things that is worth something.
When you factor in shipping, taxes and the basic tooling that the owner likely added, then to buy new it would probably cost you around $5000 (from Grizzly $4100, + $300 shipping, + vise ($170 for a 4" Grizzly, $550 for a real Kurt vise) + misc tooling + sales tax).
Your particular needs play a part too. Once you get into a budget range of $2500+ you have options. There are new bench mills complete with power feeds and DRO for less than $3000, and while Bridgeports tend to command a premium, you can often find used examples of other brands of large-ish used mills for $2000-3000 if you have the space for them.
