a: Unfortunately, buying copied x-box games where you dont own the original game is illegal.
b: The likelyhood of a burned game working on your xbox is unlikely as it takes a while to find out what DVD format and brand would work on your particular xbox.
If you're trying to buy backups, as the process of making good xbox backups is quite daunting and complex, then I would recomend http://www.shopatchris.com I have no affiliation with them, they're just the only site I know of that offers an extensive collection of xbox backups.
backing up xbox games isnt that hard really, especially with the forums and tutorials that are avaiable, if you follow the steps and dont rush it, youre fine,
I'll second that.... The tutorials sound quite daunting but I have found with Flash FXP, IsoCraxtion, ethernet card and crossover cable - Bobs your uncle!
PS3 160GB Internal Western Digital Scorpio WD1600BEVS
XBOX v1.1 with 120GB Seagate and Samsung SDG-605 --- Matrix with evox M7
PS2 39001 v7 --- Messiah 2 Pro
DJGeoJoe -d(-_-)b-