Assassin's Creed is a game that was much more interesting before than after the release. As already pointed out, much hype, and of course the ingame footage looked amazing. However after it was out, people soon realised that beyond the visuals, AC was pretty shallow in terms of story presentation and gameplay variety. That, together with the lukewarm reviews hurt the legs, even though it managed to create a surprisingly big fan base, compared with other games with similar review scores.

/edit: about the used game sales: This actually could have quite a big effect. The combination of how attractive it looks and how "bad" it was reviewed probably left quite a lot of people unsure wether it's worth buying or not... so as a compromise, they bought it used. This idea is even supported by the fact, that it had such a big first week/month, but generally disappointed people - so soon after release, there were a lot of used, cheaper copies to get, because a relatively high percentage of the "early achievers" (which were many anyway) wanted to get rid of it.

I think the game had alot of hype, and sold alot, but then word of mouth wasn't too good, plus Ubisoft released Prince of Persia a year later which was much better, maybe that killed some sales?

Thanks for commenting guys, the number of votes has nearly doubled in the last hour because of people posting. Still, while continue to feel free to talk about anything related to the thread, I would really like some more people's opinions on whether or not they think used copies effected the sales of the game.

Yeah it has great sales, i hope ACII does good to.

This whole thing happened to me. As many people in internet were dissapointed with AC  i was not sure if i should buy the game. So i get a preowned copy some months after its release. I remember going to Game and Gamestop and AC was BY FAR the game with most pre owned copies to sell, along with Motorstorm. Still today i see lots of AC copies to sell. Anyway it is a solid game, just not as good as it looked before being released. Just hope AC2 takes a step forward and improve the series.