How do I achieve the Arms Dealer Trophy in MW3?

You have to buy all attachments at least once as well as the weapons. Buying a Red Dot (or any attachment) for one gun does counts for all. One of those dedication type achievements.

If you're struggling, make sure you have each type of grenade launcher too.

Good luck!

Source & Little more Help

You only have to buy the attachments once, if the attachment is the same across multiple guns. This means that you can buy the Red Dot sight once, for one weapon, and then you won't have to buy it again on another gun.

The exception to this is the grenade launchers, which are different on 3 ARs:

• AK47 (GP25 grenade launcher)
• M4A1 (M203 grenade launcher)
• SCAR (M320 grenade launcher)

You must buy all three of the grenade launchers for this.

You don't have to do this in a single survival match, it's cumulative. Keeping track of what you've bought can be tricky, however. You'll need to make it to level 50 before you'll unlock everything. Some suggestions I read include:

• Buy a whole class of weapons out at once, so that you know you've gotten them all.
• Get a whole bunch of money (it would take $124,000 by one user's count to afford everything at once - yikes!)

Note that you also must buy the weapons you generally pick up or start with. The Five-Seven is the starting pistol, so buying it might not be obvious, and you'll generally find the 1887 in the first round, and some SMGs and ARs are dropped by enemies in later rounds.

I have not yet gotten this one myself. I looked at some guides in order to obtain this information, but I believe it is reliable given the number of independent confirmations. Some of this is detailed in this thread on Xbox360Achievemvents, and a pretty good guide is over here on TrueAchievements.