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For month of Feb according to ICv2
Just something to look at as it does not reflect the sale of the overall manga sale of feb.

9. MEGATOKYO VOL 3 TP
10. FRUITS BASKET VOL 7 GN (OF 14)
12. TRIGUN MAXIMUM VOL 4
17. RUROUNI KENSHIN VOL 11 TP
23. BATTLE ROYALE VOL 11 GN (OF 13) (MR)
31. RANMA 1/2 VOL 29 TP
33. GTO VOL 23 GN (OF 25) (MR)
38. KARE KANO VOL 14 GN (OF 18)
39. DNANGEL VOL 6 GN (OF 8)
42. TRIGUN ANIME MANGA VOL 1 TP
44. SNK VS CAPCOM VOL 1 SVC CHAOS TP
49. GRAVITATION VOL 10 GN (OF 12)
50. AI LOVE YOU VOL 7 GN (MR)
60. LEGAL DRUG VOL 2 GN
61. CHRONO CRUSADE MANGA VOL 4 TP
64. WOLFS RAIN VOL 2 GN
67. SAMURAI DEEPER KYO VOL 11 GN (MR)
70. INITIAL D VOL 16 GN (OF 26)
72. GUNSLINGER GIRL MANGA VOL 2 TP
73. KILL ME KISS ME VOL 5 GN
75. ANGEL SANCTUARY VOL 6 GN
76. GET BACKERS VOL 7 GN (OF 26)
80. HOT GIMMICK VOL 8 TP
81. PEACEMAKER KUROGANE MANGA VOL 3 TP
82. JING KING O/BANDITS TWILIGHT TALES VOL 3 GN (OF 6)
85. VAGABOND VOL 19 TP
97. ALICHINO VOL 1 GN
91. HIKARU NO GO VOL 3 TP
97. SUIKODEN III VOL 5 GN (OF 7)
98. BOYS BE VOL 3 GN (OF 20) (MR)
100. YU YU HAKUSHO VOL 6 TP

Note: The Sin City GN are selling really well (especially on amazon) due to the movie.

Where would a manga have to rank in sales to be considered as a "success"? At least in top 100? 150?

Well, these are only the specialty stores (aka Comicbook stores), so even if a manga isn't very high on this list it still could be a hit in bookstores (which now make up the majority of graphic novel and manga sales). As for what dictates a success, I have no idea. Diffrent companies have diffrent standards.