I think there are many variables when buying and selling a site. The first thing i would look into is the traffic and see how old the site is and go through the history of the traffic to co inside with any google updates.
The things i would take into account are;-
Quality of content
backlink profile (are they rented or permanent / natural)
websites age and history
Where does the traffic come from. i.e. google rankings and links
domain name value (exact match).
How many new real money players its sending.
The affiliate accounts all depend on age and how many active players there are.
If all of the above looked ok then i would look to buy between 6 to 12 months, i may go up to 18 months for very special websites. Otherwise with a google update around the corner you could soon loose most if not all your traffic.