A problem has always plagued me. Several game masters I have known have accepted payments to alter the game. In my mind this corrupted the integrity of the game in most, but not all, cases. Some games were just one on one games, so trades were made from game to game. Some where just short fun games with little integrity to begin with.
The problem seemed to be that it gave players an unfair advantage because they have money to spend. It overshadows a players hard work put into the character and effort in the game. After all, why bother when you can buy your way to the top?
Then comes the conflict. Buying gaming books and materials is expensive. Plus the situation where a player can negotiate something better is an unique and interesting one.
My solution is to create a system for barter, rewarding players for putting in the extra effort, cleaning up their area after the game and other things. The goal is to keep it low powered enough that I won't worry about players abusing it or it derailing the game, however keep it rewarding enough to keep it interesting.
The design goals are fairly straight forward from here. I want players to be rewarded for good role playing and being good guests in my home. I want players to feel they can use it to get things that are obtainable but not normally in the scope of what they'd see. I want players falling behind with other incentive programs ("homework", blog posts, etc.) to feel like they have other means of catching up. I want players that miss sessions to be able to catch up in more ways than one. Finally, I need to be able to tweak it so I can use it easily across multiple game systems.
Most of the remainder of it is just writing it out and letting the balance be found.