This is interesting... why would the new properties sell below GV?
Because, in property, the land appreciates while the building depriciates. A new building is like a new car, depriciates the day it is taken out of the showroom (or its equivalent). If I want to pay 400k for new construction with fancy kitchen and bath - I want it to my taste, not the builders.
So it is true that, conversely, ie, 50k land 450k building, should not attract much above GV?

I wouldn't buy it at any price. Its better to buy a shed in a good neighbourhood and get it done to your taste rather than to buy a fancy building in a dump. This point goes beyond monetary considerations and extend to safety and comfort of your family.
What do you mean by leaving the bid open ended?
Have a finite time period within which the seller needs to reply. This leaves you free to bid for other properties after that period and prevents the seller from "shopping around" based on the security of your written bid.
There is a gold mine of info on this thread thanks to Avalon and others. Good Luck..