The way it works is quite simple. The server is a multithreading machine. It has to be in order to serve thousands of requests per seconds. All the requests that are received from all the players are queued up, the running order is based on the time stamp inside the packet and wait in line for one of the server's processing threads to free up.
Usually at any one time there will be several threads that are freed up to take up new requests. This is when magic happens, if and only if all of your identical requests are queued up one after another. How requests are queued on the server side is out of your control, theoretically you could try to synchronise the packets but making such tool requires specialised hardware is not worth the trouble. However, you can increased your odds by making sure your requests leave your network in a consecutive fashion. For this you will need a fast internet connection and a quality router if you use one. In addition, contrary to popular belief, you will have to do this when the number of players on the server is very low. This is when the probability to have a higher number of free threads at any one time is much higher and your queue of consecutive packets does not too get fragmented when reaching the server. (being intercalated with thousands of requests from other players)
The reason why it works is because DO does not check the uid and blocks identical requests at the beginning of processing thread. Therefore you could have hundreds of request being processed in parallel on hundreds of threads and the treads are not aware of each other. Whether is due to lazy programming, incompetence or cost saving is something I don't know. But I guess it is more easy to allow for negative uridium than to bother with such protection.
As you can see, you do not receive an item if you do not make a request. Therefore buying items without using uri is not normally possible.(unless you use bugs or hacks) You could buy stuff in the past at very low prices by using Charles to trick the server in to thinking you bought something else but is no longer possible as it got fixed long time ago.
As for the claim to make negative uri into positive one is just a pile of bull..... Just use your brain for one moment and think about it. If anyone knew a way to transform -uri into +uri would keep their mouth shut and sell DO account on order. Think about it you would buy any items, boosters, keys, logs, amo etc..with Charles and then make the negative uri back to + who would need bots. Even more people wouldn't even care about ban as they would just buy a new account and play. (only rank hunting players will be left wanting). One could easily make a -8milion uri account and post the user name and password here so that everybody can check if our fellow member was able to do its magic on the account but my feeling is that it would be a waste of time as nothing will happen. (there is a way to gain uri using charles but it never worked properly, and even in the very rare instance when it does work you gain very little, is about the daily log in bonus)
And one more thing I forgot, be careful when using Charles there is a protection in place where you get charged the uri but don't get all the items. I had certainly seen it on keys happening on one of my tests account. Therefore try your move on a test account first and check if it makes sense beforehand.
Have a nice day!
Ok, I don't really know how this should work but: