I think the idea has merit, but only if you actually had to complete tasks to get the Gamerscore. Like make it a pack that includes random things, like maybe for Halo, kill 100 enemies by sticking them or in Skyrim, kill 150 chickens or something like that. Make packs of additional achievements and sell them for $5.
Yea, they do, it's called DLC (Downloadable Content), addon to games and sometimes they have extra achivement points depending on how large of a DLC it is.
But for buying points, that would make the whole system pointless. Achivements are Achievements, you get them for passing a point in a game or hitting a mark someplace. Your gamer score is based on the games you play. By buying them, it would flaw the whole system.
If you want to spend $3 for 200 achevement points, you can. Search the achievement sites, find a Windows Phone game with very easy achivements and spend the $3 in the marketplace and PLAY IT TO GET THE ACHIVEMENTS !
Or play the game for the story, I love multi-player but, I spend $50-60 on a game, I want to see the story that the programmers created the game for, then kill a few noobs....
Maybe it's me but, I really enjoy the story of these games, to me, it's the main point of playing them...yes, even black ops 2 has a cool and fun story with it...