From the FAQ:
Point values are calculated by a patented algorithm that takes in
consideration multiple factors to match the current market value for
each game. In addition to game specific factors (e.g. MSRP, age, genre,
platform etc.), Goozex dynamically adapts prices depending on supply
and demand for each game. This makes sure that if you offer a game that
is highly requested, you earn a higher value for it. Point values range
from 100 to 1000, with the most popular and newest titles near the 1000
point range.