Though to answer your question, Densk, the Union doesn't actually sell to people; it provides goods wholesale. The Trade Federation does the selling. When it comes to selling arms, they can only legally sell to those with licenses- normally planetary Governments and security forces, although many of the guns you can buy on the open market can trace their origin to the Union.
Xeth, you just need to go over what you were doing before you came here, and why. It is important you remember why the bulk of the army cannot be on Charaeus- two important reasons in fact.
Build it secretly where? You need enormous facilities to build these things- the industrial capacities of planets. One of the points of Marcus/Vlad's plot last Episode is how they saw that the secret facilities of the Union/Federation were being shut down after Naboo (and everyone involved in creating them being killed off).
The Cult doesn't have the industrial capacity to just create droid armies from scratch, hence the Union is supplying them this way.
This is very true, Densk, and it is certainly an area you have been looking in. Yes, you do already know everything I am alluding to here; it is all earlier in the thread- two very important reasons why the bulk of the army cannot be on Charaeus, which actually lead to the heart of your plot and explain why you are in this depot in the first place!
(It is also why you didn't go to Charaeus with the Jedi).
Right now, there are too many nearest planets to be of use.