Ok I'm going to start with a small note of caution (full instructions on this glitch are below):
I never used this trick during my first playthrough, and I would strongly advise against using it on your first playthrough as it may ruin the experience somewhat. One of the fun parts of the game, is gaining souls to progree your characters level, if you have an "infinite" amount of souls, then this target is lost.
This trick (if you want to do it) might be best left for NG+ and onwards, where you have seen the game already, so might be fun just maxing your character out.
In this video I use Pure Spiderstone as an example, but it will work on all stackable items (including Demon's Boss Souls).
1. This only works on stackable items, so it will work on Demon's Souls, Items, Upgrade stones etc, but not on weapons.
2. You will need to have the Evacuate miracle or a Shard or Archstone to allow transportation back to the Nexus.
3. Thomas CANNOT have any of the item in his stock, so if he does remove it.
4. Also doesn't work if you have more than 1 of the item. So if you have 3 for example, drop 2 of them, then proceed with the below.
1. Start by approaching Boldwin, and start the process of buying any item. Do not close this dialog, instead walk away towards Thomas until this dialog disappears.
2. Once it disappears, activate the Evacuate miracle (or Shard of Archstone) and when it asks if you would like to return to the Nexus, simple press "Start" to hide the dialog (DO NOT press x on that dialog).
3. Speak to Thomas, and deposit the item you want to "duplicate", when the counter option appears keep it on 1 and select "OK".
4. You will now start transporting back to the Nexus. Return to Thomas to see many more of the items you originally deposited.
5. If you did this with Demon's Souls, consume all but 1 of them, and repeat the process for a never ending supply of souls.
Let's Game in HD