My Logitech MX518 has a total of eight buttons detected by xev. Only five are usable in TF2. All buttons are detected by the game, but mouse4 and mouse5 are duplicated across some of the buttons. Here is the output of xev after pressing all mouse buttons:

ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 439631, (42,94), root:(108,675),
    state 0x10, button 1, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 440407, (42,94), root:(108,675),
    state 0x10, button 3, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 441111, (42,94), root:(108,675),
    state 0x10, button 2, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 441775, (57,87), root:(123,668),
    state 0x10, button 11, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 442631, (57,87), root:(123,668),
    state 0x10, button 12, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 443359, (57,87), root:(123,668),
    state 0x10, button 10, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 444191, (57,87), root:(123,668),
    state 0x10, button 8, same_screen YES

--
ButtonPress event, serial 36, synthetic NO, window 0x4400001,
    root 0x295, subw 0x0, time 444879, (57,87), root:(123,668),
    state 0x10, button 9, same_screen YES

And here is a xev to TF2 translation mapping for the buttons that are duplicated and therefore unavailable for unique functions:

xev 11 = mouse5
xev 12 = mouse4
xev 10 = mouse4
xev 9 = mouse5
xev 8 = mouse4

Steps to reproduce:

1. Connect a Logitech MX518 or similar mouse (NOTE: With the MX518 you need to sudo apt-get install lomoco and set LOGITECH_MOUSE_RESOLUTION=1200 in /etc/default/lomoco and reboot to get all buttons to be detected by xev)
2. Attempt to bind the buttons in TF2.

System Information:

Processor Information:
    Vendor:  GenuineIntel
    Speed: 3201 Mhz
    4 logical processors
    4 physical processors
    HyperThreading:  Unsupported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Supported
    SSE4a:  Unsupported
    SSE41:  Supported
    SSE42:  Supported

Network Information:
    Network Speed:  

Operating System Version:
    Ubuntu 12.04.1 LTS (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.2.0-35-generic
    X Server vendor:  The X.Org Foundation
    X Server release:  11103000

Video Card:
    Driver:  NVIDIA Corporation GeForce GTX 650 Ti/PCIe/SSE2

    Driver Version:  4.3.0 NVIDIA 310.14
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x10de
    DeviceID:  0x11c6
    Number of Monitors:  1
    Number of Logical Video Cards:  1
    Primary Display Resolution:  1920 x 1080
    Desktop Resolution: 1920 x 1080
    Primary Display Size: 20.51" x 11.54"  (23.50" diag)
                                            52.1cm x 29.3cm  (59.7cm diag)
    Primary Bus: PCI Express 16x
    Primary VRAM: 1024 MB
    Supported MSAA Modes:  2x 4x 8x 16x 

Sound card:
    Audio device: 

Memory:
    RAM:  7947 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_US.UTF-8
    Microphone:  Not set
    Total Hard Disk Space Available:  23465 Mb
    Largest Free Hard Disk Block:  2700 Mb

Installed software:

Recent Failure Reports: