I don't have a site for you but I can share what works in our auction league.
We can keep up to five players, provided they have not been traded during the course of the regular season. You have an opportunity to retain a player for up to 3 seasons. The first year is when you buy him. Before the second year you can keep him for one more year at the same price or sign him for two years by bumping his salary by $5.00.
For exp.- If in 2005 you bought Endsberg for $1.00 in 2006 you can keep him for $1.00 or sign him through 2007 for $6.00. It he gets traded once he is signed he must be kept by the new owner.
If I bought Endsberg in 2005 for $1.00 and traded him to you in July 2005, he cannot be considered for a keeper.
During the off-season we can trade keeper eligible players for other keeper eligible players. We can as many players for another player without a concern for maintinaing roster integrity, which must be followed during the year. For exp. I can trade Endsberg for a $21 Oswalt, a $20 Ryan, and a $15 V. Martinez.
If a player is bouth during the year to replace a waived or injured player, he can be kept at $10.00.
We found that a keeper duration over 3 years of being on one team detracts from the overall enjoyment.
Hope this helps.