buying used halo 2

hello!

so i want to buy a used game of halo 2.
what things should i look out for?
im assuming that two people with the same cd key cant play at the same time.
what happens if i i attempt to go on and someone else is using it? (worst case scenario?)
can i install the game on my computer even if its been installed before?

i dont know, anyway, id just like to know what im getting myself into before doing it. =) thanks!

hehe, thanks for your opinion. so far ive found online shopping quite trustworthy. i know i will get disapointed at times when getting an item i didnt quite expect, however the money i save is still worth it in the end. i usually pay less then half the amount of the original retail price. "Usually you can play a game on more than one computer" that sort of comforts me. i hope halo 2 counts as one of those. plus this game seems quite trust worthy, and im meeting in person so ill be able to get a good look at the product before saying okay. thanks!

any other helpful thoughts out there? =)

I had Halo 1 installed on two different computers with the CD. I found out you can't be in the same server as someone with the same key, but single player was fine and most multiplayer servers let me on. I just can't play against my brother in the same game online.